Select that face. hold right click and go to AssignNewMaterial> Lambert. Now do like you normally would to attach your picture to that lambert. The part you need is this: with that face selected, go to Edit Polygons>Texture>Planar Mapping. This will put a map of that image flat on your face. You shouldnt have to, but you may need to move that around a bit until it's on just perfect. Hope that made sense.