I used Paint.net which is an excellent free drawing and graphics program from the University of Washington. You can see where I've put in limbs and leaves. (It's kind hard to draw those leaves with a mouse!) However, this method was much quicker than drawing by hand.
Now I'm adding in the limbs and leaves on my drawing. I will need to do gray-scale of the main images to work out the lights and darks. Everything but the background will be done this way.
I like to think of this technique as the digital equivalent of doing thumbnail sketches.