Do you mean making untiling textures? Its simple. Split an image into four quadrilaterals (Preferably square, but its not vital) and the move them to the opposite side of the image. So like, bottom left square would go to top right. Then, the band-aid tool (In photoshop, don't know the GIMP equivalent) to join the seams. It should be untiling now. you can go a little further to make it look more non-tiling, but thats complicated.
