Edson, the couple of examples I posted here were just quickies made from a single png...didn't add a shadow face, sorry. Yes, to the Alan link...if you the "shaded" face style you'll see the shadow face(s) of any of my trees.
The other thing you can do is explode the png image (then hide the 4 edges), now it's just a material you can edit the color and transparency...and I'm afraid it's also a bit more bytes.
If you really want to get into it you can export the image (right-click menu on a PC...Mac?) and edit it in PS, then import it back into the component (you'll need to change the name of the new png) and erase the original.