I found a phone and there are nude pictures on it, would it be illegal to post it on the internet?

You know the answer. Don't do it.

California Penal Code Section 485: One who finds lost property under circumstances which give him knowledge of or means of inquiry as to the true owner, and who appropriates such property to his own use, or to the use of another person not entitled thereto, without first making reasonable and just efforts to find the owner and to restore the property to him, is guilty of theft. Cell phones are a uniquely capable of determination as to the owner. Turn the phone in and be without risk.

I agree with the other posters. If you do anything with these pictures, you are asking for trouble. You don't know who these people are or how they might react if they discover you distributing these private photos. Furthermore, even if you have every reason to believe these people are legal adults, if you're wrong, you will be fined, incarcerated and forced to register as a sex offender. Turn the phone in.