It actually isn't in the pivot table. That's what I meant by saying I "cheated" a little, sorry. If you move the pivot table, you will notice that the image is actually behind the pivot table, with the pivot background set to be transparent. Hope that helps!
You could "cheat" by making multiple pivot tables that are exactly the same but with the difference being the selected background picture under "Style" -> "Background". Then under "Layout" -> "Show" select conditional, and show the different tables depending on user selections.