Settings underneath Account. Tapping the word settings or clicking, should take you directly to it. There's a section called Custom Avatar, and you must upload the picture to a website. Do not use imgur as imgur and Horse Fable, has not been liking each other and causes issues. You can use img.bb or postimg.cc or any other free image hosting site out there.

Then you would upload the image, choose the proper link and you would insert it into Custom avatar text bar, scroll down to the bottom and hit Update Account, for it to save.