What I find puzzling about the photos Chad found is that the photos for Chad are implied to show evidence of Chad's infidelity, but the photo for Dawes shows Arieth's infidelity. I have a hard time coming up with a simple explanation that covers both.
Chad thinks the MC is trying to blackmail him. We know it isn't the MC, but the idea it's blackmail makes sense: Chad has a secret he doesn't want to get out. But the Dawes photos are really Arieth's secret. In theory Dawes might want to keep it a secret just to avoid a scandal, but it seems much more likely he'd either want revenge on the guys banging Arieth or he'd take it out on Arieth. Or both, of course. In either case, there's no clear mechanism for the blackmailer to profit from sending Dawes those photos.
So why were those photos sent? Without knowing who Chad was fooling around with, the only explanation that seems to fit is that someone just wants to mess with the tri-alphas. That's certainly plausible and there are plenty of suspects. But if that was the goal, why send the photos discretely? Making them public would be much more humiliating.
It's possible the sender wanted to protect Arieth's reputation, though that sure seems like a lost cause. That would imply it was one of the HOTs, but overall they have some of the least interest in messing with the jocks. The more likely explanation is that the sender wanted to avoid attracting too much attention. If the pics are public, people might figure out who sent them and that could be bad for the sender. If the pics remain private, the jocks are on their own to ID the sender. Plus, they are more likely to act privately and do something rash (as indeed happened).
Given all that, I don't see any obvious suspect. Most of the jocks' enemies seem like they would be happy with public humiliation and wouldn't mind being outed. Quinn is the only real schemer we've seen so far, so she's the only one I might buy sending the pics in the hopes of getting them to do something stupid. That might fit with her getting the HOTs to dump the jocks for the DIKs, but given Quinn's money problems, you'd think she'd settle for direct blackmail of Chad instead.
Just feels like we're going to need more information before we can draw any real conclusions.