Here is how I do it:
I created a panel on the system page called "backmain"
I then created a hidden page under the "Home" device called "Images"
Here I created panels each with a different background image and a different prontoScript name ("bg1", "bg2", etc.)
In page properties of each page I run a script to set the one I want.

var b = CF.widget( "backmain" , "PS_SYSTEM" );
var n = CF.widget("bg4", "Images", "Home");
b.setImage(n.getImage());

Is this what you want to do? I thought PEPv2 did that automatically.