A couple weeks ago, we reported on a possible Google Image Search update but that discussion phased out a bit.
Today, I spotted another WebmasterWorld thread with comments from someone I know who is obsessed with tracking Google Image search rankings. He said, besides for the design change:
Many results have changes their position. A lot of new results appears - and other passed. It is the biggest change of results since months. But until now I have no idea why. Perhaps (!) it's a change on the weight of semantic page content. Seems a little bit as onpicture factors like filename or ALT-text is not as important than before.
Maybe there is a slow update being pushed out to the image results?
Forum discussion at WebmasterWorld.