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It really is a moot point, I think, since an image url cannot "contain" a link to another url. So even if Google did bother to calculate PR for an image, that PR could not circulate to any other url.
I've done some checking, and every image url I put into my address bar shows a gray toolbar. I think the answer here is "no".
Sorry for being unclear before, but that's interesting about an inbound link straight to an image too.
If that's it, I really can't say for sure. I never link directly to image files. Even for enlargements, I always put the image in an HTML page.
Since image files aren't in the index...
But image files are now folded into "Universal Search" results on Google, so we may be in a new moment here.