OK I give up
Googled, looked through the forum, and the answer eludes me....
I want to blit an image on the screen, then draw another image on top of it.
With the second image though, I want to use an ADD rop - in other words, the RGB of the second image is added to the destination (rather than the source replacing the dest, with transparency, as normal). To make things more complicated the second image has alpha, so it's semi transparent.
Is there a nice easy render op mode I can set to do this?