I use GIMP 2.6.2 - it's a little different, but I'd think this part is the same.
Load your image.
On the top menu, go to [Layer][New layer...]. Click OK to create a new, transparent layer the same size as your loaded image.
Use the pencil or brush tool to do your outline. This drawing will be on the NEW layer, but it looks like you're drawing over your loaded image.
Go to [Layers][Stack][Select next layer]. This will make the original loaded image the active layer.
Go to [Layers][Delete Layer]. This will delete the layer holding the image and leave just the layer with your drawing on it. It will have your drawing with a checkerboard background.
Go to [Image][Flatten Image] to fill in the empty (checkboard) parts of the layer with white (as long as white is your background fill color).
Let me know if this works.