I fail to understand why are you using inkscape and don't do that directly in GIMP, which is the proper tool for photoretouching. Using vectors for something that should be pixels is too problematic (mostly for printing). According to my experience, I'd reccomend you to only use vectors if you need them, and do all the photo stuff with pixels, at the right resolution. Vector graphics are great for type and things that need to be enlarged, but they can give you some problems when it comes to interaction with bitmap images and printing. I'm not saying you can't use them, but keep that in mind. Mostly if you're using Linux, where you don't have reliable export filters for mixed vectors and bitmaps: PDF export in Inkscape isn't ready for production, SVG import in Scribus has lots of limitations (that you'll find in Adobe products too), PDFs from Scribus aren't interpreted correctly by Adobe software, and (you already found it out) GIMP doesn't have a good vectors support.