After this tutorial you will end up with something like this:
I was mainly just reading Q_Man's Pixel Stretch tutorial and crashed into this so its mostly about his tutorial.
[*]Open up any new image.
[*]Put in a render.
[*]Get out your rect. selection tool and make a selection like this:
[*]Copy and paste onto a
transparent new layer.
[*]Anchor the pasted image onto the new layer.
[*]Reset your color (D on the main GIMP screen) and get out your smudge (S on main GIMP screen)
[*]Get out a grungy brushes and smidge away on the pasted layer.
[*]Merge your layers or continue experimnenting.Other than, you done!
This is the result I got on the 2nd image I used on the tutorial: