Act 2 Scene 2 quote meaning "The sleeping and the dead are but as pictures"

Lady Macbeth is trying to calm Macbeth down. Macbeth has just killed King Duncan and is "freaking out" over what he has done. He complains that he could not complete a prayer in his head and is haunted by the dead body of the king. L. Macbeth is trying to say that the sleeping and the dead cannot harm Macbeth. She adds that they are as harmless as painted pictures.