For
kalinda These are the pics I worked with.
What I usually do is make a copy of the basic pic and cut away the heads (that's called vignette in my photoshop).
That looks something like this:
Then I 'vignette' the heads of both Avon and Cally try to match brightness/colours with the basic pic.
Then it's time to fit the heads on the bodies. That's always a hard part, because in my old version, the heads tend to get ugly (you can literally count their pixels) if you rotate them inside the basic pic. Maybe you don't have that problem in newer photoshop programs.
When the heads are attached to the bodies, I add the 'headless layer', so I won't have to clone and smudge and cover up too much with head and body.
Then I clone and smudge from the main body, so colour and structure are somewhat alike.
I usually add a transparant layer and adjust brightness and contrast untill I get a nice result. I use several layers that add colour and/or structure to the manip. That's what you see in the one I just posted. The first one is without the layer and in the second one I added this layer, made transparant 30%:
Hope this helps in your endeavours with Gimp. I'm sure there are a lot of tutorials out there, but I'm always glad to help, although I don't know if I'm of any use to you with my antique photoshop program. I have photoshop essentials but I keep putting it off to learn the basics there. It differs too much from my version (I think).