Yeah, I've been noticing the same thing. In a few of my fandoms, some kid who joined recently (and I don't want to namedrop because I'm certain this is a kid and I don't want to to put an actual child on blast) has been spamming stolen fanart into the characters' image galleries and then spam-downvoting all the actual official stuff (and also, for a period of time, spammed fictional character photos into the image galleries for their actors and then spam downvoted ALL THE ACTUAL PICTURES OF THE ACTORS THEMSELVES until I yelled at them to knock it off, and I'm still trying to clean up the mess they made.)

IDK if this is what you were alluding to, but i also know from a friend of mine that in one of their fandoms, someone's been spamming images from an old, poorly-done movie over stuff from the original source or the more recent tv show, and then spam-downvoted everything else to get the movie stuff on top. IDK if it's a troll/bot so much as it's just some very stubborn children who really want their personal favorite photos on top (even if they really shouldn't be there in the first place..), but either way at least I know I'm not the only one noticing this.