A proper discussion? All right,
I do see your point to an extent. People posting MLP pictures, with or without captions, in random threads etc is very annoying - just as anyone posting random pictures and memes in threads are. So, I can agree with you thus far. However, avatars, signatures and wallpapers? I don't get it. People make avatars and signatures of anything. Movies, games, books, anything... Why is it suddenly a problem when the image happens to be from an animated show for little girls? There are far more disturbing things in my opinion. Not liking GIFs I can understand though, I don't like them either. But that's because they're flashing and that annoys me. What is on them doesn't really matter. As for wallpapers... Well, you wouldn't see me posting random MLP pictures all over the place, and most likely not with an MLP avatar either. But if you had a look at my desktop, you'd find Rarity. But how could it possibly bother you what I have on my desktop?
About football fans, well, you don't claim they never go around wearing their team's shirt I hope? And you'll like find some that have their club symbol or something in their signature or avatar on a forum. What is the difference?
So, all in all, I agree with that it's annoying with people posting pictures where they do not belong - whatever pictures they are - but what people put in their signature is kinda up to them (within reason of course). As for posting pictures in this thread? Well, it kinda says "MLP" in the thread title, what did you expect? That's kinda like going into a thread with "Obama" in the title and be annoyed when finding out it has a discussion about American politics...
EDIT:
As for those images, yes they are retarded. I fully agree. But you are going to find A LOT of retarded and disturbing images that people make about anything if you look for it. Personally I would never have seen those images if you had not posted them here, and that is kinda the point I am trying to make. If someone used those as an avatar I would be bothered, yes. Just as if they used an image of anything equally retarded. But someone putting a picture from the actual show in their avatar? No.