Joey, don't listen to them, seriously.
You said you were using GIMP?
Then easy.
Open it in GIMP, and in the right-hand window you'll find your layers.
There should only be one layer, with your image on it.
Right-click that, and select "Add Alpha Channel".
Now, grab your Magic Wand tool, click on the white part of the image, and press either Delete, or Ctrl+X.
The white should disappear, and you should have a Gray Checkerboard pattern behind it.
Save it in a format that allows transparency (recommend PNG), and thar you go!