A Google Webmaster Help thread has one webmaster complaining his site isn't ranking well anymore in Google.
When Googler, JohnMu, checked out his site, he made one observation about the length of the alternative text (alt tags) used in the images source code. John said that the images are using "a full copy of the - sometimes long - title as the alt-text for all of the product images on the page." He recommended the webmaster stay away from throwing so much content in the alt tags of the images.
One small thing that I noticed while looking through your site's pages (well, the HTML code :-)) was that for many (all?) of your products you're including a full copy of the - sometimes long - title as the alt-text for all of the product images on the page. Keeping in mind that the alt-text is meant to be an alternative text that could be shown in place of the image (and accordingly, it's indexed as part of the page), it could be confusing to see the exact same text over and over again. Search engines generally aren't impressed by seeing the same text that many times, so I'd simplify that a bit by perhaps using the full text for the main product image, but not reusing it for all of the smaller detail-images. I've added a link below for more information on using images and alt-text.
Smaller changes like that are particularly noticeable when the pages themselves do not otherwise have a significant amount of relevant content on them, so a different strategy could be to just - as mentioned above - work on making sure that those pages have more great content on them in general. In the end, the goal should be to make it easier for search engines and users (when it comes to images, think about those with screen-readers) to understand your pages without getting confused by unnecessary duplication.
Keyword stuffing your alt tags is never a good idea.
I'd always recommend keeping the alt tags short and make sure they describe the picture.
Forum discussion at Google Webmaster Help.
This post was written earlier and scheduled to be posted today.