Good metadata tags for images will tell you what is in the image, so you can search on those terms. Unfortunately, it is time consuming to create metadata at that level of detail. This article talks about research that is being done in Europe to create software that will search within images for specific things. The software has learned to recognize specific shapes. Although the article talks about this for search engines like Google, it would be useful for digital imaging projects.
Thanks to Julia Schult at SUNY Cortland for pointing out this article to me.