I tried GIMP when it was posted on the main page. Couldn't make it work for reason. I couldn't get my pictures opened so I could work on them.
Hmmm. I haven't really had any problems with the GIMP (but I usually use it in Linux). The Windows version was kind of a pain to get installed when it was first released, but the new version is pretty quick and easy.
I'll help you get it working if you either post or PM the specific details of the problems you're having. The GIMP has the color selection, and masking tools you'll want to crop out the plane. An example of some compositing can be found in an assignment webpage I had to do a few years back.
http://www.vr.clemson.edu/~arakel/815index.html Check out the compositing project, scene 2, which specifically deals with compositing a LEGO minifig into a picture. It's not great, but shows the general idea.
Also, a tablet can really make the painting/selection part of image editing much less tedious. It can take a little while to learn how to use a table effectively, but it's a lot less strain than a mouse.
If you really want to get serious about compositing Apple's Shake software is some of the best I've used, but it's expensive ($2999, it's really meant for film/movies, but it's also great for single image composites but not editing) and only runs on Mac OS X and Linux (there was an older version for Windows). A local university might have an educational license though (that's where I've used it).