Omg so like I am so grossed out and dont know what to do..?

Im 15 and my cousin is 18. Last night he slept over cause I was home alone. He fell asleep and someone called him I answerd and then was like im gonna look at his pictures, not expecting anything. about 3 pictures in I saw one of him naked and with a *****. I have had so its not the body parts that bugged me but I didnt want to see my cousin naked.. like he has nice abs and stuff but like Ewww? Ha.. so what can I do to get that crap out of my mind. and should I tell him?