Dynamic Images Problem (Dont know what im doing!)

Im developing a flash peice for someone. They spit out a text file full of variables and variable information, one set is as follows:
Surface can also equal Grass

How do I use this variable string to choose (load) either a Grass or Dirt background image in my flash movie.
The point is is this text file will have constantly changing info, so it could be either one.

Hope I explained this well enough!

var myLV = new LoadVars();

myLV.onLoad = function(success)
          trace("error loading text file");

     if(this.surface == "Dirt")
          //load dirt background
          //load grass background

So how do I actually load the dirt/grass graphic?

assuming you want to load the background into an empty movie clip called myBackground