So, I tried changing my avatar picture. When I upload the picture and save it, only the small image in the top right hand corner changes. When I go to a different page, the image goes back to the cat I have. Does it take a long time to change or is it a bug? Please help, I seriously want to change my avatar picture.

Have you tried clearing your cache?

The same is happening to me D:
Whenever I change my avatar, the one in the top corner changes, but when I go to another page, it is back to my old one.

Yeah, it's a cache issue. Clear it and you'll be fine.