We know that they are the authors of the letters sent to the jocks, and therefore that they had access to DIK's computer to retrieve the photos. So that's not a red herring.... This nerd shit is going to be another red herring isn't it...
But as for the photo of Katy, and the video of Jade sent to Burke, we don't have 100% confirmation that it's them. Indeed, between sending anonymous letters (but not so much because it's done in such a way as to incriminate the DIKs) and publishing a teacher's nude and sending the video of the MC and Jade to her husband, there's a big difference in objective and method.
Magnar's stated aim is for the frats to destroy each other, and that's what the letters are for. If that doesn't work, he talks about a plan B. I imagine that plan B consists of destroying each frat individually, for the jocks it's enough to make Chad's sexual orientation public, for the DIKs it can't be Jade's video because it's not canon (but must be able to find something else), and for the Preps it can be anything related to Tybalt.
It's not totally crazy to say that not everything is the nerds' fault. Maybe there really is a traitor in DIK...