Have you seen this ----->User Profile Picture on Tableau Server?
The setting is documented, so yes, supported.
Below is a part of the above Link:
I did a bit of testing. When my test environment was on 10.4, no pictures for users displayed. After upgrading the environment to 10.5.5, pictures for AD users display. I was also able to click on my profile picture and upload a photo locally, which now displays instead of my AD picture.
I tried setting vizportal.adsync.update_system_user to true, restarted Tableau Server, re-synced my groups, but the manually updated photo is still there, so the setting did not bring in my AD picture again to my profile.
My concern is different here. Im using 2018.2 and I able to set the profile picture from local. No issues with that.
Thing here is, im preparing a user dashboard and would like to populate the user profile image who ever open the dashboard.