I need to create a 2d tree for a background image. I use GIMP for 2d and Anim8or and Blender for 3d. It is just going to be the background in the game, but it still must be detailed and look very good. I tried making a tree in 3d by rotating leaves along branches, but it took forever. In GIMP I can never seem to get the right look. The tree is not evergreen and should look like an Oak or Aspen. Any suggestions are appreciated.

What kind of view do you need? I'm no artist, but I believe a photograph of a tree is two-dimensional, and should provide a perfect resource.

I just need a landscape or front view of a tree, but I need it to look cartoonish for the game. A picture is a good idea and I have a few, but I don't know how to make them a cartoon image.

There are a few fractal tree generators for Blender, and in fact I bet you could find models of actual trees that you could just import. You might not want to use them just as-is, but it'd probably be easier to tweak an existing mesh than to generate one from scratch.

As for making it look cartoony, Blender has a toon shader mode that you can use.