As mentioned, the UI used kgPanels to create those textures you are seeing. Each is a frame with a background and a border. Probably found in the SharedMedia addon - kgPanels can use that media for its artwork.
To create your own, follow these instructions:
http://forums.wowace.com/showthread.php?t=15439
http://www.wowpedia.org/EdgeFiles
Put your textures in the SharedMedia_MyMedia folder and follow its instructions inside the SharedMedia folder.
Pick your new textures from the kgPanels options.