Brian,
I assume you would create a custom (button / drop down / popup) which could call a function like:
function setBackground(url) {
//Update editor
var editor1 = document.getElementById('<%=editor1.clientid%>');
editor1.GetDocument().body.style.backgroundImage = "url(" + url + ")";
//Remeber selection by saving to hidden field
var field1 = document.getElementById('<%=field1.clientid%>');
field1.value = url;
}
Ady