Be sure to add your keyword phrase into your images Alt Tag and also try to rename your images so the filename of the image will be your keyword phrase.

This is very simple to do inside WordPress media gallery and you can also use plugins that will do this for you.

A great plugin that can do this for you is called SEO Friendly Images. It is a Wordpress optimization plugin which automatically updates all images with proper ALT and TITLE attributes. If your images do not have ALT and TITLE already set, SEO Friendly Images will add them according to the options you set.