1.) Open VRChat.com
2.) Log in on an account with a VRC+ subscription
3.) Click your profile picture to open your profile.
4.) Click the pencil to edit your profile.
5.) Click your profile picture
6.) Set a custom image or upload a new one.
7.) Click Confirm
8.) Observe there is no UI to unset the picture and revert to no custom image (simply shown currently worn avatar)