heh i've been working on a theme using GraphicConverter, not as pretty as photoshop but can get the job done.

In GC I use the magic wand tool on my background, lets say it's black, choose Edit->Invert Selection, then choose Edit->Fill selection with foreground Color (making sure the foreground color is set to white first), then I do Edit->Select All and Picture->Invert.

So the most reliable method is to put your image on a black background, fill your image in with white and invert.