As someone who pirated it once (and has purchased because I liked it…long story), I’d recommend going the GIMP route. Photoshop is a wonderful program and everything, but if you’re just going to use it for this it may be too powerful for what you need. Imagine buying a gaming laptop to play minesweeper. That’s kind of what you’d be doing. Granted, it would leave you open to expand, but I’ve heard wonderful things about GIMP and if I can save you from going the illegal route, I might as well.
Also, klswoot uses Photoshop Elements last I heard. She and I spoke about it during the Invention derby, which was a while ago so she may have upgraded although I doubt it. She sounded pretty happy with it and the other programs she was using. You may want to ask around to see what everyone’s using first, but I think GIMP and Inkscape would be good places to start. There are tutorials on the internet for them just like there are for the Adobe programs.
Also, if you’d like to talk to other derby-goers, you could post in the Derby thread. Or for a bit more informal (and often off-topic) discussion there’s the Derby Cyberpub.