The leaf shapes are just standard .car files. They're in the Plants subdirectory of the Data directory. I think you can have pretty much any vertex model as a leaf but C does like a stem to attach the leaf to the tree. The simplest method is to open leaf.car and edit that, saving it with a new name in the same directory. It will then appear in the leaf shape dropdown of the tree editor. You can apply a texture map to it using the layers list shader.