Legal Question in Criminal Law in California

If it's a school portrait or an innocent vacation photo, there is nothing illegal. If the picture is sexual in nature, it might be considered child pornography.

Assuming the 16 year old girl (ladies don't send pictures like that!) sends a pornographic picture to the eighteen year old, it depends on what he does with it. If he encouraged her to sends it in the first place, keeps it to look at it later, or forwards it to his friends, he could get in big trouble. If he immediately gets rid of it and tells her never to do that again, he stands less of a chance of prosecution.

Of course, an 18 year old would be committing a crime by having ANY sort of sexual contact with a 16 year old.

Mr. Marshall is right. The 16-year-old girl's actions cannot amount to a crime by the adult. But if the photos are sexual in nature the adult could commit a crime either by encouraging her to send them or by knowingly keeping, viewing and/or distributing them.