Can't change desktop profile pic in Monterey

I had the same problem and I thought of where the photo might be used and realized it'd be in "Contacts.app."

I went in "Contacts.app" and changed the profile photo and saved then tap "done." Voila! Worked! I hope this helps your problem as well :)

Dressage24 wrote:

How to change desktop profile pic in Monterey -- my user profile will not appear to the right so it can be changed (even after unlocking).

Change a user's login picture on Mac - Apple Support

This more then likely will require a reboot or two...if cache files are not flushed try a Safeboot