In 8.20.1, Jira system administrators are now able to change a user avatar through the following steps, but we do not have this feature documented anywhere:

1. Go to Administration ( ) > User Management
2. Click on the user whose avatar you wish to change and select View Public Profile at the top right of the user screen
3. Click on the Avatar image and it should bring up a screen to allow you to select a new avatar or upload a file.