Check out my Ultra Slideshow Tutorial video.
At the 3:26 mark delete the other entries in the UltraSlideshow.xml from the !-- Background -- table so that you have this.
At the 3:46 mark delete the other entries in the UltraSlideshow.js from the "var background =new Array" table so that you're left with
var background = new Array (
new Actor ("bg01")
, new Actor (bg02")
Change var random =Math.floor (Math.random( )*15); to
var random =Math.floor (Math.random( )*2);
That should do the trick.