I'm glad someone mentioned these!
I've never personally used GIMP, but Paint.NET was what I used to get into GFX, then I moved into photoshop.

I had another program, I can't remember its name, and it was a mad piece of software because you had to type in numbers instead of using your mouse, which was very appealing to me.
Photoshop kind of offers this, but not as well as this software I had.
It also allowed you to easily make animated .GIFs.