In 3D terms you could look for "billboards". This is term used when its just a picture on a flat polygon in your scene, to speed things up.

A billboard is going to have an image of a tree with an accompanying black & white cutout which in the 3D program is used to remove everything on the tree image except the tree, so it will look natural.

These can be used in 2D programs as well as you can use the black & white image to do the same basic operation.

Anyway, Here are a couple of links with some free examples of billboards. They are zipped and the images are .tga, which carry the black & white cutout image in a seperate layer.

http://www.marlinstudios.com/samples/sampvtf.htm

http://www.marlinstudios.com/samples/samptrop.htm

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