Thats so cool Tom.
I have many self made trees that I have in 'clip-map' form, I never use .png as they are currently not recognized in Podium, however with the new V2 coming out soon that will support it I have started converting some so as to be ready for beta testing.
I have some great trees that are like a zillion MB that I have posed in another app and rendered them from many different angles so that I can extract the alpha map perfectly and create crisp .png's from them. The advantage to doing this is I can take a tree like for an example a Magnolia and render 4 different rotated views of the same tree and make 4 different .png's from that tree, one can then use 4 different 'billboards' in mass creating a forest or mass of the same species without getting a pattern and it all seems so natural.
Like the pink blooming tree above, I have 3 other views of it that can be placed next to each other without looking duplicated.