Some people are really really scared of doing something on their web site that would be considered search spam. So those people ask some interesting questions.
I personally found this question pretty interesting, especially since a Googler responded to it.
A person asked in the Google Webmaster Help forums if he can use the same alt (alternative) text tags to describe the same image but multiple size options. He asked:
I am in the process of adding alt text to the images on my website. I like the idea for accessibility and for better exposure to google images. Each piece has three views- main view-second view-and a detail. There are larger versions of each that get called up in a lightbox. All 6 images have unique filenames. I am at a loss how to properly manage the alt text for the images. If the alt text was all the same or similar what would happen in the google images search? It seems kind of obnoxious to have 6 images show up for one search. That said it seems kind of spammy targeting different keywords for each of the 6 variations.
Peter Linsley, who was formerly with Ask.com, now helps lead up the Google Image search team, answered the question. He said:
The same alt text for different sizes/representations of the same image is just fine.
So no, this is not considered spam.
Forum discussion at Google Webmaster Help.