Changing my image type to jpeg seems to have done the trick, but I want transparency, so I'm gonna see if I can get it to png.
EDIT: Okay, so it seems to revert back to the old image when I use my png image again, so I tried gif and it seems to have worked.
I'm guessing the website becomes confused when the old and new image are the same, so when a different type of image is put in, they're no longer the same. Thanks for the help!