It should be a transparent gif if it's anything other than a rectangular image. When I create transparent gifs. I save them with a matting (the edge area of the graphic) that is close to the same color they will be covering: in this case, a shade of green from the field.
You could save a graphic as a transparent gif without any matting applied and it will work, but it might look a little more "jagged" than one with matting.