The idea of painting the pixels is the research part. How do you do it? One idea is to have a specific brush shape preloaded, and use that as a mask, moving it across the image and displaying pixels as it moves. Then the user may get the idea that they are “holding” the shape and cup their hand appropriately.
Trouble is, the arm and other parts of the body will also be moving, so you have to do a good job of picking out the appendage!