Need help making dornas

[Anmino]

Hi, I've been trying to make my personal dornas and to practice making patches, I decided to start practicing making dornas.
I have a problem making this picture http://i54.tinypic.com/2e1defo.png.
How can I put those 4 pictures in one lay out? That layout has 512X512 pixels of measurement and I think that's the default size of it and it should be the same sizes of all dornas to run it in game.

I'm using GIMP as my photo editor. Need help please. NEed some advice. Please