MINOR??? It may be lots of things but minor is not it.
Still, if the owner did not have possession of the computer for a length of time, he has a defense.
And pretty obviously, some other people are using his computer!
1) We don't know if this was one picture or more.
2) Students download tons of things.
3) Someone else may have downloaded it onto his computer.
Do you turn in everyone you know to be breaking the law, or do you discuss it with them first? If you know someone who's mixed up with drugs, do you try to convince them to go to church, AA or rehab, or do you call the police?
This is minor in comparison to murder, rape, theft, or arson - felonious acts that absolutely should be reported.