Do links which have no text inside the anchor (just an image inside the A tag) have the same value to Google for the page rank?
I think it should effect SEO than PR. Giving an Alt attribute may better optimize your page than keeping it without it. Should not effect PR.
The amount of misinformation in this thread saddens me. I assume either I have misunderstood your question or everyone else has. If you're talking about an image as content on a page, it won't affect PR at all because only backlinks affect PR. If you're talking about an image hyperlink as a backlink to your page, it will affect PR.