The easy one to hit on in this process is the caption. Your caption can be your main keyword or a very small variation of the keyword to identify what you are sharing with the reader. I almost always just use the main keyword as the caption as I am stressing that point to the visitor that actually reads it.
There is a place to give your image an alternative name called "alternative text". This is where I will either use a synonym or even go and use a longer tailed keyword. Do not give it the same name as your caption as that is wasting the opportunity to get more keywords in front of the bots face and thus missing out on some free easy SEO.
Nest place to consider when using images on your site for SEO is the description. Here you can really ride the wild stallion and add in 150 - 200 words about the image. I come real close to keyword stuffing here and, although what I put makes sense, I am it worried about it being Pulitzer prize material. I want to give the bots something to consider and take back to Google central so they say, "Hey, the guy from yPick.me has a pretty strong site, let's rank it!" so there is some meat in that part of the image backend.
