Disturbed, I have been through this with my own DD recently, who not only sent pictures but also a video of herself to boys at school. You must, I repeat must, contact the school as soon as possible to sort this out. I shied away from doing this only to have the whole thing come back and bite us again a few weeks later. Fortunately in my case the boys involved destroyed the pictures and did not pass them on, but even so word got out about what had happened and my daughter went through a horrendous few months of bullying and being ostracized at school as a result. She still has to put up with a few ignorant boys making lewd comments whenever she walks by, but thanks to the school's actions, and the passage of time, this is mercifully gradually stopping.
What I have learnt is that these things will never just go away, there will always be some nasty little sod who will throw it back in her face. The boy involved in your daughter's case is breaking the law by having these pictures, and I'm sorry, but so is your daughter by sending them. You really need to talk to her school, they will almost certainly have come across this before and will know what to do. They will also be able to make sure that any pictures are removed from the boy's PC/phone - just hope that it's not too late.
Like my DD, yours is probably suffering from self-esteem and confidence issues too, and this is really not going to help her. Believe me, any humiliation she may feel from you going to her school about this is nothing to what she will feel if the pictures get circulated. My daughter is now having counselling, arranged by her school, and it is really helping her. I feel I failed as a parent in not realising how low her self-esteem was and allowing her too much freedom with the internet and her phone. She is no longer allowed to use her phone or laptop upstairs where we can't see what she is doing. I only wish I had done that years ago.
Your DD needs you to act to protect her and help her deal with her low self-esteem. I completely understand how upsetting and difficult it is but please please do not wait, contact her school straight away.
Let me know how you get on. I wish you and your daughter all the best, you have a difficult time ahead of you but this issue will not solve itself, you need to be strong for your DD now. My relationship with my DD is now stronger and closer than it's ever been, and I'm sure that yours will be too.
Good luck!