be sure to set up your shot first. Use the time slide to slow the film down when you get close to your shot, pause the film, use the sliders to get the best angle of your shot, hit the "fullscreen" button to get rid of the buttons and lists and such, ans most important... MOVE THE MOUSE CURSOR OFF THE SCREEN !
Many a great shot has to be re-done when you find that stupid cursor is in the way.
then follow Bloozes instructions. Gimp is a free art program that you can use if you want to mess with the pictures more than MS Paint allows.