The real problem is quantities. While I agree that one picture of Shinji strangling Asuka on a ruined beach wouldn't have spoiled me, if I saw dozens of them, when there's a common theme that spreads across multiple artists, that's when it becomes pretty obvious that something is a spoiler.
What I think is more likely is that, if, say riffraff, stumbled across that picture and liked it so much he posted it here and then the mods came in and said "That's a spoiler" and removed it. riffraff hasn't spoiled himself so he wouldn't know that it's a spoiler when he posted it, but he would now. Unfortunately there's nothing that can be done about that, though.