Knowing how to select parts of your Gimp documents with the help of Quick Mask might be one of the first things you should learn when starting with Gimp. Let me introduce you to the basics of Quick Mask.
If you found yourself in a position where you wanted to crop a certain photo into a perfect square but you just couldn't figure it out how, than this tutorial is for you.
Pretty nice looking and simple to create effect, where we turn everything into pixels except the selected part. My car will end up looking like it's trying to escape out of the pixelated background.