Those are beautiful drawings, even if you did cheat a bit. Maybe I'll try that soon.....if I can figure out how to work the program.
Ok, this isn't really working like I had hoped so I'll just tell you the trick.
All the good drawing apps support layers. Import what you would like to draw into one of the layers. Then switch to another layer and draw over the original picture until you have a good outline. Finally delete the layer with the source image inside and fill in the details and shadows of your drawing.
With just a little technological help, anyone can do it!
Another tip is to zoom in a lot, and use light colored pens.