Thumbs are done in Photoshop. No fancy tools. Using only a square brush with Opacity, flow, size set to pen pressure and Angle jitter set to Initial direction. Same for eraser. Trick I use is, zoom the canvas right out so the image you are painting on is this size in the photoshop window and use a large brush size.
I also have the navigator open for a larger view but I try not to look at it much. Brush to add silhouette, Erase to add highlights. Thats it really.
Heres another I did this evening. Time 1.5hrs