I also want to be able to do this. If it is not yet possible, please submit this as a request. I think this would be a huge benefit for Wix website admins.
Again, we want site members to see the option to upload a picture, but also be able to choose from an optional “Avatar” library provided by the admin. This is such a great idea!
The workaround might work, if someone can give a step by step on how to do it.