does he know its from your phone? are u recognisable in the photo?
if he doesnt say anything SAY NOTHING.
if he says something, and u r not recognisable and he knows its from ur phone,
and it is a contract phone, and u can CHANGE YOUR NUMBER AND SAY IT WAS STOLEN.
if u cant change ur number,
a) does it look like "real" porn? if so, say u lost ur phone at a party and someone picked up ur phone and sent porn to all the numbers. b) if it looks like a real pic, but u r not recognisable say someone at the party knicked it and took dirty photos with it.
someone then handed in your phone.
NO MATTER WHAT YOU DO, DO NOT APOLOGISE, after all, it wasnt u eh? pretend u r pissed off about losing ur phone and someone using the credit on it to send dirty photos.
c)if its clearly of u, say that u had a medical condition, hemorroids etc and wanted to see if someone had any ideas what was wrong.
d) FORGET IT AND ASK YOUR DAD IF HE HAD BEEN DRINKING when he received this photo as clearly it couldnt be u. i do not think he will keep the evidence.