Google has long showed a list of related search terms when you use its Image Search option. A new enhancement now shows thumbnails of those related images when you mouse over the links.
Like most Google enhancements, this is being rolled out gradually, so it might be a while before you see it on your account. I'm in two minds about the usefulness of this (I rarely use the related search option in the first place); what do you think? Good addition or a waste of bandwidth?
Helping you find what’s in the mind’s eye with improved related searches [Official Google Blog]