If your user account is at the role level of Admin/SLT, then you have the ability to edit the details of other users’ profiles. Including the ability to change the avatar photo (profile picture) that accompanies the posts that they write.
Changing the Profile Picture
From the Dashboard click on ‘Users’ > ‘All Users’ in the left hand menu, or hover over your username (top left of the screen), then click ‘Edit my profile’.
Browse through the list of users until you find the one that you want to change. Hover over their name and click ‘edit’
On the profile page, scroll down to Avatar
Click on ‘Choose File‘
Browse to select a photo from your device, click ‘Update Profile’