A specific search will reveal all Bing thumbnails indexed by Google. If you click on any of the results, you see the regular Image Search details page, but you sometimes get redirected to the original source of the full size image.
If you don't pay attention to the URL of the image thumbnail and not of the original site, you may not even notice that the photo comes from Bing rather than its rightful place.
The issue was discovered by Tristan Savatier who posted on Google's Webmaster Central forum. He brought it up since he found it peculiar for Google to be indexing Bing data.
A Google employee said he will look into it and raise the issue with the people responsible for image search, but there is nothing else from Google on the matter at the moment.
However, it's unlikely that Google does this on purpose, judging by past comments and actions. Google's Matt Cutts said a few years ago that users don't like more search results in their search results, for obvious reasons.
And there's an even bigger reason, a couple of months ago, Google made a big mess about Bing using Google search results, gathered indirectly from Bing Toolbar users, as ranking signals then applied to its own searches.
It would be rather ironic for Google to be indexing cached Bing images, even though it's not exactly the same thing. It is a bit strange that Google is indexing anything with bing.com in its domain, but the issue will probably be fixed soon enough.