Well, Nick being responsible is technically a theory as we don't know who leaked that Cathy pic on the DIK's account.
My money is still on Quinn as part of some scheme to distract the DIKs while she looked for their money stash that she probably found out about from Tommy.
That just doesn't make sense.
Quinn distracted the DIKs so they'd leave the laptop unguarded so that she could use it to post the Cathy cluck which would in turn force the DIKs to run around frantically searching the mansion which would inevitably force her to call off her own search long before she could find anything? Really? Quinn has demonstrated she's no Moriarty, but even she can do better than that. Causing a highly public scandal that disrupts the party is anti-helpful.
*If* Quinn is behind the Cathy cluck, it has to be because someone paid her to do it (Dawe being the most likely suspect). That would still derive from Quinn's known desperation for money, but avoids requiring insane troll logic on Quinn's part.
But troll logic or hit job, either way I doubt Quinn is behind it. Episode 5 was all about Quinn selling out the DIKs in a desperate bid for money. Tommy has a big confrontation with her over it at the end of the episode. If Quinn was also behind the Cathy cluck, then we're going to be stuck repeating the exact same story beats whenever the truth comes out. What would that get us, other than making Episode 5 look even less significant in hindsight?