You can't do this if your on a mobile device,
I've been having the same problem, it's a real bummer....
Click on "my profile" on the upper right of this screen. That brings you to your profile page, and on the left is your avatar or the generic placeholder image for an avatar. Underneath it you'll see a link that says "replace avatar"... click that. Then you can browse your computer or the Internet, pick an image, and insert it.
If you don't have one that you want to use, go to this thread:
https://www.backyardchickens.com/t/712889/need-an-avatar-lots-of-free-ones-here-take-your-pick
You'll find loads of free avatars to choose from.