Take the picture or painting off the hook and place it in a safe location.
Push the top and bottom metal parts of the hook together with your fingers and turn the hook 180 degrees upside down.
Slowly pull the hook out while twisting it carefully to avoid damaging the wall or enlarge the hole in which the hook lies. Pull it until it is completely out of the wall.
Put a small amount of plaster in the wall to cover up the hole in which the hook stayed.