Picture is the same sign as photograph. Take your dominant hand, shaped in the form of the letter 'C' in ASL, and hold it to the side of your eye. Then hold up your non-dominant, open, flat palm vertically, and bring down your 'C' hand to touch the palm. It is as if you are indicating that what you see is being "captured" on paper, in a picture.
Teach your baby this sign when it's time for a baby or family photo op, or when you are looking at pictures with your baby.
Teach your baby the picture sign to start teaching him that images of people, places, and things can be captured on paper. It's nice to teach alongside art as well.