A related tip is to render your png image OVER the background you want to use-- THEN save your render as a png file. Render the same size as the background you wish to use. This way you can 'turn on' anti-alising. The alias'd edges are a very small area in comparision the the rest of the image. And it makes the colors blend better. And eliminates that sharp and jaged appearance and blends in with matching colors.