Even suggesting that it could be Rusty is stupid. It's not Rusty, Derek, or MC; and Jacob is highly unlikely.
Most likely, it's either Nick, Tommy, or none of them. I will be disappointed if it's just Jamie's lost phone though and that really doesn't explain a couple things. (One is that the laptop WAS stolen during the party; and two - not that they're necessarily related - but someone installed a hidden camera in Derek's closet a long time ago -- I think before Jamie lost his phone -- and had delivered the photographs to Chad.)
Here's the exact sequence of events:
1) Someone sent Chad a letter with images and a letter with the words 'Is this what you're hiding?' And then Chad fights MC and then shows the Tri-Alphas the images of Arieth in the closet with the DIKs and said that's what had been sent to him when they ask. (Also, the photographs taken of Arieth seem to have been recorded by the hidden camera and are unlikely to have been on the laptop)(I suppose it's technically possible that he was ONLY sent a photograph of himself using steroids or kissing Troy and then he lied to the Tri-Alphas and gave them the photos of Arieth as instructed by the blackmailer or to hide what was really sent but that might be a bit of a reach...)
2) The laptop gets stolen at a party and the Cathy Cluck is sent.
3) The same night that the Cathy Cluck is sent, MC can leave he party to go have sex with Jade and record it. Not long afterwards, he can show the video to the DIKs just as hell week ends. (A few days later, Rusty finally decides to tighten security regarding the laptop.)
-> It's possible that not all three events are related and there could be different culprits for all three.
-> Quinn and Tommy had conspired in the past against their presidents. Quinn is supposed to create a rift between the Tri-Alphas and the HOTs in return for a drug den at the DIK's mansion. A bunch of people are cool with this including John Boy and Elena. Worth mentioning because I find it hard to believe that Quinn or Tommy aren't involved in at least one of these.
-> Regarding the laptop getting stolen, no one outside of the DIKs is even aware of it's existence. Meaning that a DIK had to be involved unless it was the phone synced to Jamie's phone (if that's even possible, since, the laptop was in fact stolen at the party).