I think that is a red flag warning and you should discuss with the child's doctor.
I know this question is old but just in case someone else needs more answers. If the child is normal in most aspects of their particular age then it's just a picture of a dog being hot in the head with a rock. I have four kids two are boys ones 19 and one is 11. They drew stuff that was questionable maybe to others but I knew it was just a picture and they weren't psychos.
This could very likely mean nothing. But you can talk to the boy to discuss it. My son came home with a picture one of his buddies drew for him (as a gift) in first grade and it was of his friend with a shot gun shooting my son. I told my son it wasn't an appropriate picture. It's now years later and the friend is still a friend and is not in prison for homicide :>) good luck