Theoretically you could be charged (by the state, not by a citizen), with anything, but based on these facts about a photo of you dry humping and some comments, it seems unlikely that there'd be enough to pursue anything or get a conviction.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
California does not have a statutory rape law per se but instead it prohibits sex between those who are too young to consent and those who are over 21. Among these statutes is a prohibition on lewd and lascivious conduct with a child which you may have violated. This is a serious charge with serious consequences (lifetime registration as a sex offender among them) so you should get a criminallawyer to advise you as soon as possible.
I normally agree with attorney Koslyn, this time I slightly disagree - dry humping a minor at a party isn't necessarily a joke (I notice there are no ages listed - the bigger the age difference and the younger the girl is going to make a big difference). If she wishes to pursue it then there may be problems. If there is no intercourse, then no rape, but there can still definitely be a crime. If you are an adult and she is a minor and they have evidence of dry humping and she's upset...keep your mouth shut and only talk with a good criminal defense attorney. If she wants to pursue the matter and has pictures....
If you get a call from anyone, including her, don't comment! If the police want a "friendly" conversation at the station, don't do it...get a defense attorney.
To add to the other answers -
There are many other potential charges besides "statutory rape" (a.k.a. Unlawful intercourse with a minor) that are in play here. If the girl is under 14, there is Penal Code section 288(a) - Lewd acts on a child, which carries up to 8 years in prison, is a strike and requires lifetime sex offender registration.
There's also Penal Code section 647.6 - Annoying or molesting a child under 18. Even though that's a misdemeanor, it too carries lifetime sex offender registration and up to a year in county jail.
Keep quiet about this unless you're talking in a confidential setting with a criminal defense attorney. Nobody else is safe to talk to about this. Don't post any myspace, facebook, twitter or texts about any of this. It's bad enough they have the picture - don't give them any more ammunition, should this case get to the police, the DA and into court.