For reference, I have completed a module that has features I believe "User picture API" was meant to have, including caching and allowing additional picture providers via an API.

Core support for Gravatar, and Robohash is included in the core package. Support for Facebook and Twitter among other providers can be found in See more at User Pic Kit Extras!.

See more at User Pic Kit.

To Maintainer: Dave, consider this an invitation to work on the project if you have an interest.