Who keeps changing it, and why?

I'm guessing someone keeps changing it to something random or to something that's not inline with the rules?

Something like that. I can see changing it every few days, or even every day or so (though there's really no need for it). But now it seems to change several times a day, which is getting a little out of hand. 

Are they changint it to poor/bad images?

I've changed at least once a few days ago, but only because someone had changed the image to fan art. But when it comes to characters like Spider-Man, Captain America, Ms. Marvel, Batman, ect I tend to leave them alone because their main pages are always getting changed constantly. But I can do understand how it can get annoying when the image is being changed almost every single day.

Changing it every few days for variety is fine. But when the image is being changed several times a day, it starts to look suspiciously like a pissing contest between users who insist on having "their" choice on the main page. Which is ridiculous. It's time to back off, people.