For some reason this one has come up a lot lately. People asking how to add alt tags to images on their website for SEO purposes. At one time it had some value. What we tell our clients today is if you want to use Alt tags then don't put any keywords in those tags. From Google:
alt attributes with keywords ("keyword stuffing") results in a negative user experience, and may cause your site to be perceived as spam."
The short answer is that keywords in alt tags for search engine optimization purposes has no value. If anything it may hurt you. The best approach would be to describe the picture without keywords in 16 words or less.