It seems like its possible with gimp, although I use photoshop and overlay it on a different layer alittle transparent. You can probably do it in gimp to some extent aswell.

for example this was literally just a blacknwhite gradient with a face scribbled ontop, I didn't even overlay it or add colour lol. Although I left a massive seam at the back >_<

But overlay more crap and it works better.