And to answer your question, yes. GIMP should work fine, as Coup simply loads in images. All you need is an image-editing program. It isn't some type of special file being used.
Pretty much, yeah. There will probably be some positioning stuff as well as options for multiple layers, opacity, and link colors too, but at the basic level all you really need to do is create an image (or find one) and link it in the background image field.
Well I was working on it and now GIMP just flat out won't let me draw anymore so I'm pretty pissed. Oh, scratch that. It won't let me work on the Template. Just the template, everything else works -_-