well, to get more help, i will explain...
gimp is a classic program in tools, has classic effects and brushes/tools.
but in extras it is as advanced as photoshop, its basically easy for new people, and there are people still using it, so i suggest you to use gimp if you are not really skilled in real.
If you see that you are good and born with art skills, then use photoshop, its a paid program, but i think you can find it for free anyways...
photoshop is an advanced art program in everything, but need who has advanced skills.
Something funny explains it: a small warrior just started to learn, should use the sample weapons, or the advanced weapons? if a warrior have skills born with it, then advance weapons, i hope this helps you