On Desktop:

  1. Select Settings on the top-right menu under the profile image.
  2. Navigate to the Profile section.
  3. Hover over the default image until Update Pic appears.
  4. Click on Update Pic, upload your new photo, then select Update

On your Android device:

Tap on the default image under Settings and upload the desired picture.

On iOS device:

Click on Shares or Storage, then tap on Settings listed at the bottom of the page.