Changing your profile picture is easily accomplished within your Rivly account.
Be sure that you are logged into your account and go to the Profile page located in your user dropdown menu in top right of screen.
There you will see the section pertaining to your profile photos and an edit button located in the top right corner containing the ability to change this image. Please note that the image must have a minimum size of 300x300 and only JPG, PNG and GIF files are accepted.