On many projects in my carrier I was needed to handle situation when I need to show some "avatar" image for user when he is missing photo. Usually with gems like paperclip, carrierwave you creating a fallback and it returns a dummy photo.
More interesting solution is to show initials of the user in some circle or rectangle like Google (and many others) doing it.
First solution what we used is custom html/css and very simple helper. This solution is currently on one my project.