Well first off you need to start the layout designer and point to the background image using the menu background/image. Then you background image needs to be named properly. Lets say your ini file in the ../mamewah/ini folder is called 000mame.ini, you layout needs to be named 000mame-main.jpg and you layout needs to be named 000mame.lay. The entire naming convention is explained in one of the docs.
Good Luck,
John