I'm not sure of what you're asking. Two possibilities: 1)You want to make the background transparent so all you see is limbs and leaves, or 2)you misunderstand cropping versus masking, due to a lack of having a black outline around the image to be removed.
Masking is a totally different process than cropping and there is no real EASY way to do it with an image with a color palette of thousands of different colored pixels. However, the alternative options are to select images with more solid backgrounds to make this process easier. I would start with gathering more information on masking, specifically GIMP masking.
_________________
"In order to attain the impossible, one must attempt the absurd."
~ Miguel de Cervantes