Ummm, well, it just sort of happened by accident!
When I take a "screen shot" on my Galaxy Tab, the photo editing software automatically pops up and the draw feature is enabled.
I assume other tabs (iPads etc) can do the same.....
1) just take the screen shot (Galaxy Tabs its the 4th icon from the left at the bottom, and iPads I think you press the home key and power button at the same time -I know Gert mentioned exactly what to do in the Ipad tips and tricks thread)
2) open photo editing software
3) find a draw feature
4) draw what you want with your finger
5) save!
You can draw on photos using your PC and the simple paint program too. If it's useful for anyone, I can give more detailed instructions.