1. Start from your Core persona.
  2. Navigate to Users > Edit User Profile Data.
  3. Search and select the user for which you want to update the User Photo.
  4. Scroll to the System Information section and select User Photo
  5. Click the Choose File button, and choose your photo (if currently no photo)
  6. Add a caption (not required)
  7. Click Save
  8. If there is an existing profile photo you want to update, check the Delete on Save button on the User Photo screen.
  9. Click Save.
  10. Click the Choose File button, and choose your photo.
  11. Enter a Caption (optional).
  12. Click Save or Save & Exit.
You can also go through Core > People Finder > Search for the user and select them. Then go to the Contact Card and click on User Photo and update. 
Note: This can take 24 hours to display within some areas of the software

Also see: How do I bulk upload user photos?