Of course. Most of the "default" images in the levels3d/gui folder is GUI related, but its easier to go by the "guiblue_" ones for starters. That way you know all you can work with.
You upload them like any other images (dont forget to name them SERVERNAME_* !).
You can find some tutorials on how to use them in the following URL.
http://wiki.graal.net/index.php/Crea...iControls_List