Gravatar is a common feature on forum and blogging platforms, like the one this site uses (called Q2A). In concept, it is supposed to make avatars transferable across different websites — it has the advantage that you can manage all your forum avatars from one place.
Q2A does have a standard uploading system, which Pokemaster has addressed before. He used Gravatar as it was a better solution years back:
Originally I didn't use uploads because they were stored in a weird way in the software. But that has changed so it's certainly possible to allow uploads now. I’ll look into doing that at some point.
Gravatar is free service, so sponsorships wouldn’t be on its agenda. Gravatar is owned by WordPress though, so sites that use it do give exposure to that platform.