As I've mentioned many times, Linux comes with some amazing and powerful software. gimp is one such program. And Gimp has a great tutorial section at:
One really neat tutorial shows how to create custom brushes in The Gimp.
It's incredibly easy to do. For instance, I took this image of my comic strip character George and created a "George brush" in Gimp.
In a nutshell, all you do is create the brush (using Gimp) and save it in the Gimp Brush format. Just make sure this new saved brush is placed in the Gimp's brushes folder. In Linux, this folder is "~/.gimp-2.2/brushes." Then refresh the Gimp's brushes menu (or restart Gimp) and the new brush will appear ready to use.
Gimp is powerful and easy to use. and it comes standard with Linux.
Get creative with Gimp. Get Linux.