I would believe anything the GIMP guru says. RobA kind of told me how to do this in another post just a few days ago, and he used some of the same techniques in his post today to make the palm trees. From what I understand, you make 1 layer for each individual thing. So, if you want 10 squares at different sizes you make one per layer, set them all visible and save it as a GIMP pipe brush (or whatever). Then, when you select the brush, pencil, or airbrush tool and that "brush" as you stroke, it will cycle between the layers to spit out the image on said layer. Not easy, but it's once and done. If you want to test it, just to two layers with drastically different shapes or colored thingies and then give it a try. I hope to make a few copies with different colors of my hand drawn mountains in the next few days and post them up for everyone to use(assuming I get it done and assuming it looks ok).
Joe