I've been using GIMP to make my ink drawings transparent, and superimposing them onto photos or paintings. Here are a few examples.
Haystacks (and Hut) in Winter
My shepherd’s hut found its way into Monet’s Haystacks, where it doesn’t seem entirely out of place.
Signs of life in Uyuni
I superimposed my transparent drawing on a photograph of the Uyuni salt flats (Bolivia).
Pen and ink drawing of the great trash vortex superimposed on a picture of a clear lagoon. Background photo by Eray Ersen.
I cropped the original ink outline to remove the eyes, just kept the wavy hair, and added transparent blue to the sea life. Background photo by eliminator on http://www.wallpapersdb.org.