S.G.
I have read th responses and to be honest I have to disagree with them. First of all, he is a senior in high schoo, soon going off on his to college. He has already made the choice to keep the picture and not delete it. At his age which is probable very close to 18 he sees himself as an adult. I am guessing the girl does as well. You may be shocked to discover that he and she have done more than send pictures.
If you called the girls parents or talked to the girl you would start a process of alienating your son at a time when most boys his age cant wait to get out of the house any way.
That does not mean that what is happening is right, but you may be shocked to find out how common it is. What you find disgusting is now seen as a way of flirting by those that have already been intimate with each other. Girls and boys alike do this type of thing without realizing how public the pictures could become. I am not sure that either of them would call the pictures disgusting. We live in a different world with different values now. The kids today have friends with benefits, they hook up together without feeling any kind of connection to the individual. It is more about the sexual experience than a committment. That is why the call it friends with benefits. You can have a group of 8 friends, 4 boys and 4 girls, with no romantic involvement. however they may have no problem hooking up for the night, there are no strings attached for any of them and they go back and forth between one another.
I am not saying this is what is happening, and I am not saying it is right, i am just saying it is what happens in a lot of cases.
your best bet is to talk to your son, openly. He may not be as open with you, but he may. Ask him about the relationship, find out why she would send that kind of pic. If you are willing to talk about and find out about what may be going on he may be willing to discuss his relationship openly. At that point you can try to reach him with common sense, but most kids at this age go through a stage in life when they know it all and their parents live in the stone age.
My youngest is now 21, and I have one who will be graduating college soon. The ideas and lifestyles of young people are very different today. It takes constant communication to make a point and keep the lines open, constant.