There is a very helpful thread in the Forum: Forum Bulletin The post is titled: Modify Avatar-How to upload an image as your own Avatar
I can not post links, but this may help you, it helped me.
I try every day to change my avatar but it doesn't work. If I'm understanding from reading other posts, this is an issue that has been around a long time. Can not complete tasks because of this issue.
I have problems with the size of mine but haven’t gotten around to changing it from upload but someone suggested trying to do it on the computer rather than on tablet or phone