How do you create cloth texture for computer game?

The basic garments are supplied and you can color or alter them. I downloaded PhotoScape but cannot figure out how to use it or even drag the files over. First I tried Gimp2 and before using it I got a warning of a virus in the download. Are there any free or cheap basic programs to use? Nothing elaborate just something to "make my own clothes" that would be easy for a computer illliterate to use.