Joking aside if we're speculating my money would be on Sarah. Sarah is friends with the dorm girls who judging both by Beth and the girl telling Tara they have something good at the party are the ones behind the DIK and Cathy pic bullshit if I remember right Sarah is at the mansion when the laptop went missing. Sarah is also one of the handful of characters that know Chad is gay and as you said leads MC to a camera. Could all be a big ball of nothing but would hardly be surprising if Sarah is revealed to be shady either.
I don't think it'd turn out to be Quinn so much as Sarah doing her own thing though.
Yeah, Sarah was in the mansion when the Cathy cluck was posted, and from what we can tell... wasn't in the main party room while they were playing "spin the dildo". So, it's possible she could have been involved. Also, in the subsequent free roam, when the MC starts questioning people about what they know... when he talks to Sarah & Mel, they basically tell him to not play dumb... because they assume he posted the cluck. A guilty person would certainly want to shift the blame, and try to pin it on somebody else.
Now, if it was one of the girls at the party... at the time of the Cathy cluck posting, Quinn, Josy, Maya, Lily, Arieth, Ashley & Elena were all in the middle of the room spinning the dildo. There were random dorm girls in the background, then in the free roam see Heather, Becky, and a hammered Mona upstairs... as well as Riona (sitting by herself) and Penny & Christy (in line for the bathroom) in the hallway next to the library where the laptop was found. The only other girls not accounted for are Sage (no way), Camila (later babysits drunk Mona), and Sally, Karen & Beth... who we don't even know if they were there or not.
So, out of all the girls, Sarah, Mel, and then just by location... Riona... are the suspicious ones to me. You could include Penny & Becky as well, but at this point in the game, we don't even know their names yet.
Sarah & Mel did know that Chad was gay. So, how did they know? Would Chad have trusted them enough to let them know his secret? I doubt it. Why would he let any HOTs know? Those two seemed to be close to him at the HOT party, but I'm thinking it due to the Chad/Sage situation... not the Chad/Troy one. So, other than Sarah, Mel, and Troy... who else knew that Chad was gay? It certainly wasn't an Alpha... or the DIKs... or Sage... and whoever dropped the blackmail envelopes (we never actually saw what was in the one with Chad's name on it) off at the Alpha's house, hasn't yet exposed his secret. Unless we find out that there's somebody else who knows about Chad & Troy, the prime suspects have to be Sarah & Mel... possibly with the help of a dorm girl or two.
In addition to frequently hanging out at the DIK mansion, Sarah & Mel work out at the same gym as Chad & Troy... which apparently is also the gym that Cathy was at when here nude photo was taken, so it is plausible that those two could be behind all of it... the blackmail, Cathy photo & cluck, and the DIK pics being hung up all over campus. Add in that cryptic conversation they had with each other at the pool when Sage apologized to them for thinking they were fucking Chad... I'm really starting to think those two bitches are up to no good.