I think it would be better if the user downloaded the image, made adjustments in the paint program he or she has (or give instructions on how to do that with a webbased paint program) and then uploaded the changed picture again. That would be easier to build than what you are now trying to achieve. Unless you can find some ready made java paint program.
Instead of drawing you could divide the human body into little sections and have the user click on the sections that they felt pain. Then based on where they clicked, you could change the background color of that section.
Here's a link to get you started, but it would be a good amount of effort (but very cool).
If you are using V5, might it be possible to create a specific layout with that picture as a background and position marker-boxes over possible areas ??
Take time and write a widget for jquery based on wpaint , it will work