poor eunhyuk gained nothing from that
Hmmm... that depends on 'gain' - but let's leave personal stuff apart.
SuJu was already on downhill by 2012, and he was not the most popular member of SuJu either. So apart from fans going crazy for some time (Which anyway happens every few months over something or other), it did not have any longstanding impact on him.
I subscribe to the theory that it was a planned stunt.
For IU,
In an TV show just a week or so before, the Senior Singer was talking about watching porn and relationships etc., (the show also had Ga In and Suzy)
She had been repeatedly talked about her dislike of being cast into the pure, innocent younger sister image.
With the photo upload, she perfectly destroyed that image and then her creative control on her career grew by bounds and leaps. She took a massive risk, but it paid off.
For Eunhyuk,
Maybe he wanted some controversy to boost his flagging career, and took the same risk. But that did not happen for him. So no gain, but no particular loss either.
Of course segments of ELFs believe that he was innocent and the slyfox outfoxed him. But I doubt that, because IU remains on good terms with other SuJu members, like Heechul and has a generally a good reputation.
So either it was truly an accident or it was a calculated move, which worked for IU very well, but meh for Eunhyuk.