Gimp Graphic Software
Gimp is a fantastic graphic software. It has a gazillion of functions.
Fortunately you do not have to learn all of them to use it.
Eventually I will have a tutorial on this site on how to use the functions I find the most useful.
In the mean time you can download it here
Many tutorial exist on the web, but you can get started with the sections "beginner", "photo editing" on this page.