If you want to create textures, buildbuttons, wireframes, etc. and you cannot afford Paint Shop Pro, I would recommend The Gimp. It is more powerful than Paint Shop Pro and can convert most common graphic files, for example .bmp to .tga.The Gimp is a Linux- based graphics software and it is absolutely free. To run The Gimp under Windows, you need to install the Gimp Runtime Environment first. You can download the Gimp here: http://gimp-win.sourceforge.net/stable.html
I have been aware of that program and its good, but its extremely old now! (since i heard of it there have been several newer versions)! But still, it is a good program (as you said) to use for photo-editing, it is a little more complicated than Photoshop / Paint Shop Pro to use but once you get the hang of it (which i haven't yet) its worth you while. I use Photoshop CS, but i have used the Gimp previously as to when i didn't have Photoshop or and photo-editing software! but still, it is pretty complicated.
Carefully mentioning warez Ask. You don't want to drop yourself in it.
Its a good program if you can get the hang of it. IMO Photoshop is easier to use if you have a few hundred pounds burning a hole in your pocket. I beleive that you can download the latest versions from the GIMP home page, which is www.gimp.org
Sorry for above comment, i was being sarcastic, not intended it to be taken that way!