I was under the same impression as seodude stated. I usually tag images, but thats to help contribute to the focus on the page and to possibly rank in an image search.
I am actually running a test on this as we speak. We all know that google counts the first link on a page, so set up two test groups, one I used anchor text to link to a page and then linked an image to the same page, in the second group I linked the image and then after it put a link with anchor text.
Test Group 1:
(Anchor Text Link) (image link with tag)
Test Group 2:
(image link with tag) (Anchor Text Link)
I’m hoping to be able to track any PR shift and increase in ranking for the obscure terms based on this test.