The following suggestions may be best suited for an add-on, but I wanted to get everyone's impressions:
1. Give members who upload images the ability to specify a shirt title or description (limited to a certain character length) for the image and inser those keywords in the ALT tag for the uploaded image.
2. Give administrators the ability to specify the keywords to be included in the ALT tags for images uploaded to ALL Forums or allow this to be specified on an individual forum/subforum basis.
3. Alternatively, automatically insert the meta keywords (limited to a certain character count) in the ALT tag when an image is uploaded.
If any of the above points run afoul of SEO whitehat techniques, then please disregard that point. We don't wan to promote blackhattery. More importantly is point #1 and just the main idea of ensuring that no uploaded image goes without an ALT tag.