I don't know if there's a specific option to do that, but I guess you could copy someone else's default avatar and upload it via the normal means.
Just delete your current avatar.
You can do that from “My Avatar Gallery” on your profile.
I don't see the delete button
Do you see a check box that says "delete" right below your avatar in your avatar gallery? Check the box, then click the "process deletes/shares" at the bottom of the page.
No, just the share button
Maybe the delete button is inside your avatar.
The delete and share button are the same button, but you need to check the "Delete" box.
"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard FeynmanThere is no delete box
Okay, I just had to use desktop to do it (I was unable to do it on mobile for some reason)
I don't want my avatar anymore, how do I remove it?