I could never keep quiet if I saw evidence of child abuse. What are you doing about it, OP???

As a caregiver, you are most likely a mandated reporter. This means you report even a suspicion of abuse. A suspicion. Talk to someone at her school about how to go about reporting it. I would recommend against taking any pictures, as that could cause more trouble for you than its worth. If CPS investigates they will interview you, the child if she is verbal, and her parents, and you will have a chance to explain what you saw that concerned you. They don't expect you to play detective.