Wow this started in 2002 and last reply a few days ago.... I guess things get answered a little slow LOL (joking)...
I had the same problem. It looks like a black square net around the tree. I opened the tree.bmp file and selected the black. Then deleted it and filled it with pure black (all values 0). That fixed it. I think it may be that someone didnt make sure it was pure black when they made it.
Hope that helps,
Mike