I’m so sorry you have had the shock of your life over this.
I regularly deal with this kind of situation at work. I would advise, that if at all possible you contact the recipients’ caregivers or parents and ask for the images to be removed. This may be difficult but it is important that the young people see that the adults have noticed and are transparent about what has happened. The recipients‘ parents may be full of righteous indignation, but rarely in over 50 cases I have dealt with, do young women send unsolicited pictures and the parents of the boy need to know what has occurred, what is in his possession.
I know this is contentious, but please sit your daughter down and ask for all passwords etc and systematically go through her accounts, preferably with her. Has there been coercion? Is there pressure? What has she been accessing online? As distressing as it is, discussions now can help her make better choices in the future.
As a parent you need to be gentle on yourself, unfortunately many young people from the best of homes, with the most loving parents, see this behaviour as normal ( I know, she will have had warnings and education, but in the heat of hormonal privacy things often go awry and with devastating consequences).
As adults we are dealing with a change in societal norms for our children because of technology. I don’t believe the educational/parental messages have any sway in the privacy of teen bedrooms when they are looking for acceptance and or in the first flush of hormonal infatuation. Now is the time for you showing love, discussion, transparency and taking control as much as you can despite the probable howls of teenage protestation.
I feel for all parents who find this, hot chocolate and plain speaking for your daughter, probably a stiff drink for you once she has cried and is safely asleep. Damage limitation, love and acceptance she has stuffed up but will survive is your way through.