Is it cheating and what do I do?

I was on my boyfriend's phone because I was going to send myself a picture cause he has loads of pictures of me that he screenshots and saves that I don't have and I needed I new profile picture... then I see a screenshot of a Snapchat conversation and it was two ass pics, it was just the but nothing else no face or anything... I confronted him about it and he stayed quiet... he's is one to hardly show his emotions but he started crying and apologizing saying how he didn't know who it was etc. and the only reason he kept it was because he jerked off to it... he keeps begging me for a second chance and Everytime we see each other he begins to cry and moves away cause he doesn't want me to see him.. at this point I don't know what to do... I have never second guessed if he would ever cheat on me because if I want to use if phone or something he doesn't hesitate and nothing before has ever made me feel like he would cheat on me... I just don't really know what to do... he now constantly tells me that he knows he doesn't deserve another chance and that he doesn't deserve me but he just wants one more chance to prove to me that he'll change and at any point or time I feel like he doesn't change that I can leave him and he'll understand that he really fucked this up

side note we've been together for about 9 months
Is it cheating and what do I do?
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