I agree with Travis here. I'd also add that it's important to use a wide range of descriptive language. i.e. summer dress; floral dress; vintage skirt etc. I find a richness and variety of language works really well rather than focusing too closely around a single dominant keyword like 'dresses'.
Build useful ALT tags for users who can't see images, rather than search engines, and I don't think you can go too far wrong.