Create cracks in your images randomly using default Gimp filters.
A tutorial on extracting a person and their hair from the background.
Fast Forward past the first 45 seconds of this video.
Learn how to use the iwarp filter for squishing, stretching or growing images in weird ways. Good for making funny faces.
Take multiple pictures of yourself in a scene, then combine those pictures to create the effect of being multiple places at the same time.
Design your own personalized wallpaper with your own text and free art from the internet.
Combine pictures together to put eyes on your hands.
Make characters look like they are sticking out of the picture frame.
Use layers and the Path Tool to create some nice word art.
The best tutorial on making realistic looking tattoos on skin (or any surface). This works with black/white and color tattoo's.
A short lesson which may inspire you to create similar looks for your images.