What's Behind It All
What do you do if you have a nice photo of a person, animal, or other image, but the background is ugly or distracting, or you're really interested only in the main subject? Well, the first thing you can do is remove the background. (See Which Tool Should I Use to Cut Out an Irregular Image?) But then what? Now you have an image just hanging in space.
This novice-level tutorial explains one technique you can use to create a textured background for your image by using various GIMP filters and overlaid layers.