Paper that works well for this - diplomas, citations, stationary, fancy copy paper. Simply scan the paper and crop a perfect square. Make it kind of small. 1' x 1' works well. Harder Wallpapers Find a geometric image to scan.
Bricks work well as does lined paper.
Scan the picture. Crop the picture so that the items on the ends and top are cut exactly in half. That way they will line up when posted as wallpaper. This is tough. Try doing one on lined paper first for practice.