GIMP is a very powerful editor. It's open source and free (legally
). It can do nearly everything that Photoshop or Paint Shop Pro can. You definitely want to learn to work with layers and alpha channel transparencies.
Plus, there's a
'portable' version of GIMP that can run off of a thumb drive or memory stick.
I started off with Paint Shop Pro several years back, so the user interface was a little tricky to me at first, but once I got used to it, I use it all the time with no problem.