
The first and at one time largest of the microstock libraries. Now part of the Getty images empire. Since 2003 we have watched the cost of photos steadily rise (good news for photographers), gone are the days when you could get a good usable image for $1, they introduced XSmall images and in effect the cost of an image tripled overnight. In early 2010 exclusive images doubled in price making istockphoto very attractive to those who are willing to tie their images to just one agency. In 2014 Istock dropped selling smaller sized images at lower rates and introduced flat pricing per image no matter what the resolution; overnight the price of our 'standard sized 2mp' image shot up by 50%, but of course buyers get the full resolution for that price.
Overall the site is still popular among corporate/business buyers, in 2013 istock opened to doors to what a lot of photographers feel are inferior images, they (at time of writing) accept pretty much anything; regardless of bad keywording or image quality - istock is still however picky in terms of intellectual property. In doing this istock have been able to massively increase the size of their image collection in the last year, the dowside being that long-tail searches often reveal lots of irrelevant, falsely titled/described cruft.
You should include iStockPhoto in your portfolio. Don't lose any sleep if as a non-exclusive you earn less on this site than on some of the other smaller sites.
The Killer Search
From all the work that goes into disambiguation of uploads istockphoto should have pretty much the best search technology ever seen at a microstock agency. Sadly the implementation leaves a lot to be desired and it's clear that some other financial or business concerns are seriously eroding the quality of search results. The search technology used makes it slightly more difficult to upload to as you have clarify the meaning of your keywords. for example include a keyword 'orange' and you will be asked to tick one of two boxes Orange (Descriptive Colour) or Orange (Citrus Fruit), or perhaps you include the keyword 'lead' the response will be Lead (Metal), Dog Lead (Pet Equipment), Leading (Moving Activity), Graphite (Material) etc. and the list goes on for 1000's of different ambiguous words in the English language. This really is cool stuff, as an image purchaser trying to clarify exactly what sort of image you want using words is a difficult business and this helps significantly.
Exclusive Photographer? 
A big decision for any photographer to make. Becoming exclusive with istock will mean that you cannot upload images to any other site. I can't comment on what it's like to be exclusive as I have no experience, many people swear by it. Exclusive photographers get a larger percentage per image purchase, and also receive 'care' from the site - istock will chase people who use your images in breach of the license on your behalf. You will also come across 'Exclusive+' which allows exclusive photographers to nominate a small amount of their images for sale at a higher price.
The top exclusive photographers also get to sell their images via getty's image network (including photodisk) increasing sales potential dramatically.
Audio, Video, Ilustration, Music, Flash, Logos... 
Along with several other agencies istockphoto also accept video, vector illustrations, and submissions to their audio library 'istockaudio'. Even if you only contribute photographs to istock it's important to consider that buyers may find istock a convenient one-stop-shop for all their media needs, and these buyers will be your potential customers.
It's true that at present many buyers are only in the market for images, but demand for video is growing fast and royalty rates are higher. I can see one recipe for success in this emerging market is having quality video with matching print resolution images, hence designers can create matching online video and web/print campaigns; perhaps even just the option to go back and download matching images if they become needed will be enough to make your video work stand out.
istock Tips 
Uploading
Istock is the only major microstock site that does not support FTP/SFTP upload. There are a couple of work-arounds for this. Firstly there is the istock image manager available for download from their site (istockphoto.com/ws_client_intro.php). The second option is to use picworkflow or similar upload assistant to submit images but make sure you log into istockphoto and set those categories and keywords once the upload is done. The third option is a simple time saver: bookmark the page that is shown when you have clicked “upload > image type” (the one with a browse button) put the bookmark in a shortcut bar in your browser, then once you have logged in you can click it and save yourself a couple of clicks and page loads for every image you want to upload i.e. just click that shortcut when you receive the upload confirmation page and you are ready for another image.
Promotion
Number of images estimate: Getty/istock are tight lipped about the number of images in the istock collection.
Conclusion
Despite my criticisms of this site, the low royalty level (a low 15% for non exclusives, compared to previous levels of about 28%, no way to sugar-coat subscription rates for non-exclusive, they are disgraceful), the fact that it has fallen from the #1 spot, tedious keywording process etc, I still recommend istockphoto as one of the top three earning microstock agencies. The microstock market moves and changes all the time, although slipping they still hold 3rd place
is it still possible for amateurs to join?
Ronnie Hall (not verified) on Tue, 2009-11-10 11:26Certainly not a top 10 agency
Jab (not verified) on Sat, 2010-11-13 16:58Not quite sure how this
Steve Gibson on Sun, 2010-11-14 23:59Not quite sure how this comment fits into the middle of the thread about joining, but with the sales figures as they are there is no way, no matter how much you dislike it, that it is not a top 5. They definitely belong up there, measured on sales, images, contributors and any other metric I can think of - including photographer ire.
I have to side with you on the issues after F5, it's was all decidedly shaky with broken links, ophaned pages etc, and dodgy search for several days most of that is sorted now?
no website that big is ever perfect, especially in those dark corners. Poor interface? - well Controlled vocab often gets peoples back up because, well, yes it's tedious at first until you learn the way it works. (personally I didn't like the new search at all).
Joining
Steve Gibson on Wed, 2009-11-11 01:58Bad
Henri Faure (not verified) on Tue, 2009-11-17 13:04don't don't
Steve Gibson on Wed, 2009-11-18 02:47Exclusive Contributors
Alistair Shankie (not verified) on Sun, 2010-01-10 19:00Used to be good, now the pits!
Anonymous (not verified) on Wed, 2010-09-15 14:57iStock - not again, not ever...
Heater (not verified) on Tue, 2011-03-22 06:25istockphoto search is no
Alex (not verified) on Mon, 2011-04-04 22:18istock search
Steve Gibson on Fri, 2011-04-08 04:52I hear you alax, I've already been fairly vocal over the past few years about how the search at istock has been gradually whittled at until its become (as of 2011 for me) unusable. the results are indeed a mine-field of premium priced images that you can't sort by and istock have made it clear that there will be filtering/ordering features to allow sort by price. its a real shame, with all the keywords disambiguated the results should be excellent, and indeed are better than most if it were not for the various ill motived annoyances.
e.g. try a search between shutterstock and istock for "camera book" (i.e. a photo of a book and camera to emote learning or books about cameras etc) at time or writing on SS all you get is a page after page of books with not a photo with a camera in sight, istockphoto at least does show me a photo of a camera sitting on an open book - perfect!
It's only when you use search in earnest that you really see problems arise. Personally iI've stopped buying from IS long ago, SS is only acceptable if you are tied to that subscription otherwise it would be very temptng to go somewhere else, searching images is not easy
Istock files acceptance
Santhosh (not verified) on Sat, 2011-12-03 16:19istock change
Steve Gibson on Sun, 2011-12-04 22:28istock will change when they have to - but that's not guaranteed. This is only my opinion, but from external appearances istock seems partially paralyzed by what seems like an internal morass of 'big business'. They will either change or suffer some mortification at Getty's hands. No company is too large to fail and the larger they are and the less in touch with customers and contributors they get more likely that is. Once trust has been lost (for many that is already the case) then it's very hard to win back.
BUT... (isn't there always one) the large contributors with bargaining power to create bulk image submissions, bargain for higher royalties etc. can more than supply istock with 'most' of it's fresh image needs, meaning 'they can do this'.
It's interesting to hear this from a newbie(if you are?), most experienced microstockers, tho perhaps upset at times, look at their income from istock and, perhaps begrudgingly continue uploading; things are always changing and falling istock incomes might mean a revaluation for many. For me its the little things that annoy, a seemingly 'well controlled' amount of free speech in the istock forums (bans, locked threads etc) troubles me deeply - and would from any business.
Best Microstock sites
Pie (not verified) on Mon, 2012-04-16 14:02Vector Sites
Steve Gibson on Tue, 2012-04-17 00:36I'm not a vector illustrator so I'll highlight that first.
Theres a growing trend in crowdsourced content to have single marketplaces that sell all kinds of media (video, still and vector, and perhaps also audio, templates, 3d models, even scripts and code) so with that in mind think carefully about specialist vector-only sites and those larger agencies with an established client base who sell photos and also vectors - the later might be just as good if not better
I have a list of microstock agencies that sell vectors in vector format
also have a look at microstock blogs and sites - theres a couple that specialise in vectors/illustration.
Finally even if they don't accept vector illustrations have a look very seriously at shuterstock for selling rasterised versions of your work.
istock
Anonymous (not verified) on Thu, 2014-10-09 06:32change
Steve Gibson on Sat, 2014-10-11 00:59Over the past few years istock has transformed itself from the hardest but best selling place to sell your images (i.e. strict reviews) to opening the flood-gates to everything. I hear what you are saying about quality now, the istock collection is now peppered with bad photos; wrong, spammy or plain misleading descriptions and meta data - it was only 2 years ago that their collection was the best curated, tightly managed in the industry. i'd like to think that they have some secret-sauce technology that is working to protect buyers from the bad images through historic views, reputation and sales - but sadly I don't think that is the case.
This change (less than 18 months ago for the review quality) makes reading information about istock difficult, espeically when what should be quality references like books on the subject are now out of date. There are still references to 'istock' the leadng microstock site, shuttestock took on that role several years ago.