I'm assuming that you're trying to make a 2d billboard of a tree?

If so, you need two images.  The first is a picture of the tree.  Then you need a mask (the same picture of the tree with white where you want the parts of the tree picture to appear and black where you don't want it to appear.

Then, put the tree into the color channel and then put the mask into the alpha channel.  You can also do this with the transparency channel but it's a tweense more tricky.

If you still get nothing, there's a chance that you're looking at the plane from the wrong side.

Try that and let us know if that works.

-Kix