Well GIMP is a nice software when you already made your textures and want to add/change colors.
It is also quite simple to learn it, you have so much possibilities that you should not look on tutorials because they may show you things you don't know.
The best way to learn using GIMP is setting yourself a goal. try to reach the goal with searching on the internet for the functions that you need.
If you ask me then better use Inkscape for creating images. No matter what anyone tells you, its the best.