We rarely ever talk about Bing's outstanding image search, but I spotted a thread today in the Bing Community forums on that topic.
In short, one webmaster complained that Bing had about 50% of the images from his site, when compared to Google. So Google had about 50 pictures from this guys site in their index, coming up for a site command. Bing had about half that.
Brett Yount forwarded the complaint the the image search team at Bing and when he received a response, he copy and pasted it in the forums. This is what they said:
All in all I would say we have ok coverage for this site. We do not include all images that we know about in our index -- it is quite plausible that we decided to not include certain images from this site due to better relative ranking of other sites during crawl time. We should see its image search rank go up (and our coverage of it improve) as the site gets more popular and the images get reused and referred to by more sites.
This is really no different than web search - Google, Bing and other search engines may not consider a page worthy of crawling or indexing due to it's popularity or lack there of. This works the same way with images, according to Bing.
Forum discussion at Bing Community.