If her only damage was a head ache, I'm not sure exactly what you would try to recover from the school. Talk to the teacher/principal and find out more about what happened (how did it fall?, did they have reason to believe it would fall?, how did the school hang the picture?) Instead of a big-time lawsuit you may only be receiving an apology. Ask you daughter how she's feeling today - if she's feeling worse, get her checked out by a doctor for documentation purposes. Good luck.