[Pharo-project] Trying to combine two images

Are there any good examples of using a mask to highlight portions of an image? I have an onion-skin effect in mind, but could live with almost anything that is easy to see and does not obliterate the underlying image. The goal is to paint a mask that will select irregular regions of an image. If I have a black and white mask, I can use #eraseShape: and #reverse to get two separate images, and I appear to be able to make the masked portion of one of them transpaent and can (not terribly well) highlight the masked portion of the other. Things quickly sprial down hill when I try to combine the two pieces. Any pointers would be appreciatiated. Bill