First off, I don't think they really were blackmail letters. Chad thought that when he attacked the MC, but as he himself admits, he wasn't thinking clearly at the time. Unless the letter told Chad to break up with Sage (which is difficult to square with the Mel & Sarah scene if the MC is on Sage's path in Episode 5), Chad doesn't seem to have done anything that would take blackmail. Also, unless Chad secretly removed a separate letter for Dawe, his letter contained nothing but photos of DIKs banging Arieth. Given that everyone knows Arieth is sleeping around, that's about as far from blackmail as clandestine envelopes get.
Personally, I'm think the most likely theory is that someone either wanted to piss of Dawe and Chad, or wanted to stir up trouble between the DIKs and jocks specifically. The question then is who would do it? Troy could have been a good candidate before we confirmed he's Chad's secret lover, but he's out now. The nerds might have the capability and a general motive, but there's nothing in game that links them to the pictures. Vinny might want to stir up trouble, but it's unclear how he would know about Chad's secret. Sarah and Mel do know about the secret and clearly have an agenda, but the look between Chad and Mel make me think Chad knows they know, which would make the letters pointless. I even tried to examine the timeline once to see if anyone was suspiciously absent when the letters were delivered, but sadly the scenes at the start of Episode 3 are so out of order I can't even be sure what scenes are contemporary.
Unfortunately, I think this is one of those mysteries where we don't even know enough to speculate meaningfully. Which is definitely frustrating given it's been languishing for 5 episodes and counting.
