Nice! If you ever want a white background,(I use Photoshop, but I know how to work GIMP).
First, take a picture with white paper in the background.
Then, go to GIMP, then open a picture.
Next, look up on the menu bar until you
find the menu labeled "Colors". Click it.
Finally, you find the sub-menu labeled "Levels" in the "Colors" menu bar. Click it. Now a window with slide bars appear. Have fun! IMPORTANT: Only use the top bars! ;-)