The rub is that you already have Gravatar, which is 100% free, offers several options, and is easy as cake to implement... What compelling reason would someone

The rub is that you already have Gravatar, which is 100% free, offers several options, and is easy as cake to implement... What compelling reason would someone have to use this service (and pay for it) that they cannot already get at Gravatar? i.e. what problem are you solving, or what underserved niche of Gravatar's customer base are you targeting, and how are they better served?