Click on the avatar for you it would be blank for me it has the pic of my head. A drop down will appear with different fields click on the "Settings" field.

The page will change to "Public Profile Settings" The image you want to use needs to hosted somewhere else. Copy the link of that image and place it in the appropiate field. I like uploading images to https://imgbb.com/ and using the link they give me.

Godd luck!