The trees in the libraries often have embedded leaf images. (Embedded means that they are stored in the tree definition, but not stored separately on the disk).
You can use these trees, and their leaves, to make new trees by modifying them and saving them with a different name.
If you want to use other leaves on your own trees, you need to find a source for Alpha-transparent PNG images of leaves. I looked around on the net but couldn't find any good, transparent, png images.
Has anyone found a good source for leaves?
Here is an image of the leaf in the library poplar tree: