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