I suppose they assume that you will always be covering up the back. Unlike XY Graphs, there is no empty space within the plot region and transparency is not allowed in the z-scale colors. In principle you could broaden the X and Y scales and see the region surrounding your plot.
Transparent PNGs are not a problem for the front image, GIF is not natively supported so a few more steps would be required. You could also fake it by turning the background and decoration transparent and strategically placing a picture indicator behind the graph, or try customizing the indicator.
If you achieve a nice effect, post a screenshot, this could be one of those features I never knew I wanted. If you are the lone user of this effect than I am afraid it is kluge-city.
