Many people seem to struggle with the right mousebutton menu for some reason, but you can now step over that pitfall. Gimp is also included in all of the major Linux distributions. Gimp should also work in pretty much all the common unix systems. Layers My intention is to help you understand the concept of layers and how to use them in Gimp. This is a bit more advanced tutorial, but it shouldnt be too hard if you are familiar with the very basics of Gimp.
Layers are like a stack of overhead transparencies.
You can paint on them and the non-painted areas of a layer will show through anything that is under it. The layers on the image are partially transparent so you can see the blue layer under the red one. The partial transparency is the thing we are taking advantage of when doing our image. We start by opening a new image - make it 300x400 pixels and select the RGB mode. Also set the default fill to background and check that your background color in the Gimp Toolbox is black.