There is no compensation for this. It is part of the Revised Artists Supply Agreement we all signed:
Promotional Use
You will no longer be able to opt-out of the Promotional Use provision of the ASAs, which allows us to use our contributors content for marketing and promotional purposes without compensation. We use your images, video, flash, audio and illustrations to promote the site, bring in traffic, drive people to portfolios, and ultimately sell more content. Wherever possible, iStock will provide attribution for all files used. We want to be able to showcase the best of iStockphoto, including your files.
newbielink:http://www.istockphoto.com/article_view.php?ID=1165 [nonactive]