1. Open up your image with Gimp.
  2. Add a transparent layer by clicking on Layer >> Transparency >> Add Alpha Channel.
  3. Click on the Fuzzy Selection icon. Unless your background is fairly similar to your subject matter, set a fairly high threshold. Be sure to hold down SHIFT while making your selections.
  4. Hit Delete. If you see a checkered background, then you have your transparent background. If you see anything still floating around the background, use the Eraser tool to get rid of it.
  5. Save as a *.GIF or *.PNG, whichever suits your purpose. If unsure, try saving in both formats.

How to Make a Transparent Image Using Gimp