I do not consider him a sex offender. That said, I also do not know the technical criteria for a sex offender. If you've got it, then great. I do understand the emotional outbursts ("think of the kids!") when some is labeled as a sex offender.
If my daughter posted a picture like that, I would first start to look inward to understand why my daughter is taking a picture like that and giving it out to everyone. "But Dad!" She'll say. "I love him and he said that he won't do it!"
At this point, I'll be sad that I did a piss poor job of raising a child who is that naive and blame myself.
Will I hate the guy? Sure. Will I kill him or hurt him? Why? All he did was post up a nude picture of my kid. The killing or hurting desire is only triggered when my kid is raped or murdered.
I wouldn't pushing the child to a degree where he had a permanent record.
Fortunately, I have boys.