I've done this for our school nurse visit database. the diagram is overlayed with as many transparent boxes as you want, each acting as a button (or you can assemble a drwaing out of variously-shaped buttons, as shown. If the user clicks on, for instance, the right hand, The text "right hand" is entered into the InjuryLocation field.
(You didn't sign your post, so forgive if my guess is incorrect)
A few years ago, back around version 10, Andy Cohen,PHD wrote an article for Filemaker Advisor titled "Build data-driven Graphic Displays with FileMaker Pro". It should be available via FilemakerSphere.com if you have access the article # is19654. He used the very concept you are considering here as the example for the article. You may be able to find him directly, he also was involved in a podcast, "Filemaker Success Tips" http://fmsuccesstips.libsyn.com
Hope this Helps,