You feel guilty because those two girls thought you and your friend were single. I'll bet they even hit on you and it made you feel good. If they were your friends before that night, it wouldnt be anything, but they were strangers. Don't take advice from reddit incels who cant even get a girlfriend in the first place. Also, dont try to use the 'i was drunk' excuse- you knew what you were doing at the time (and if you didnt, you need to drink less around strangers). If you don't tell her about it, I bet the picture will appear again in the future. And when that happens, she'll know you were lying about something, and she'll think it's cheating.