I have used Canvas (Deneba Software, I think it's in Florida somewhere...I'll
find out specifically if you're interested) to create composites and overlays
on the MAC with both vector and bitmap graphics in the past. It provides
extensive bitmap editing, including bitmap region tracing, which can then
be treated as polygons of line art. Of all the graphics manipulation
software I've used, Canvas was the best at bitmap editing for our purposes,
mainly flow cytometric presentation.