For this kind of Textures (Leafs / Tree's and so on) i Use the Path tool... Select the outershape / create selection and Hit "DELETE".. voila the model has no background... (Firstly create a Alpha channel)

cYa Sebastian

Tutorials:
[Blender]Terrain creation ENG/GER
[Blender]Low Poly Tree Modeling
[GIMP]Create a Texture for Terrains
CLICK HERE