I would really prefer to keep the title (hover) text on icons short. When it's several lines (like, the length of an xkcd title text, for instance), it can be difficult to read all of at a time (it can disappear quickly), and honestly as a sighted user most of the time I don't care about either the description or the comment - if I want to know that information, I can just click through to the allpics page. Keywords, by contrast, are required to be short. Plus, since icons can have multiple keywords, it's more important to draw focus to which keyword in particular is used in each instance, as the description and comment stay constant.

My understanding of comment vs description is that comment is basically for crediting and context, and description is a textual equivalent of the image. Almost all of the time I would say that the comment will be entirely irrelevant as hover text (again, if I want to know who made an icon, I can go to allpics), and for sighted users (correct me if I'm wrong, but if I understand this post correctly, they're the ones who interface with the hover text almost exclusively?) including the description field of the icon is redundant information.

So, in summary: I strongly favor the status quo. Keep the title attribute "username: $keyword", keep the alt attribute "$description", and keep the $comment field relegated to the allpics page.