Visit Menu (three horizontal dots in the bottom right hand corner of any logged in page) > Settings (the cog icon in the top right hand corner) > Account Settings > Manage My Photos > Click ‘manage’ when viewing your public gallery (if your image is already uploaded to your profile and approved) > Select the yellow star icon on the image you would like to use as your main photo.
Please note that if you try to move a restricted image from your private album to your public gallery, you will be advised that you cannot. You cannot have a restricted image as a main photo.