overlaying a pixmap over an RGB image

[Rok Roskar <rroskar artsci wustl edu>]

I've created an RGB image from a data file, but now I want to be able to
draw things on it which result from user interaction. How can I do this?
I tried overlaying a pixmap on top of the RGB image, but it just covers
it completely... apparently pixmaps arent transparent. :)  Any ideas?

Rok