One of the most underutilized tools in the hands of webmasters who are publishing photos is the title. Both "title" and "alt" text should be used when publishing photos on your website. When you label your pictures you are making it easier for the search engines to index them. Having your images show up on the images search pages of Google, Yahoo, Live, etc. is a great way to build up your page rank and get traffic.
You should always try to include your target keywords in these labels as long as you can keep them relevant to the photo. There is less competition right now for premium keywords in the image search area so you should take full advantage of it while you still can.