Nice idea! What surprised me most: It is also very well executed, with a sane and simple API, good performance considerations, and it fits its purpose almost perfectly.
Having said that, please pardon my ignorance, but:
What's the advantage over simply not using an avatar at all?
Honestly? Nothing really. Well, you don't have to think about the code part but besides that? Nothing. Actually it would be more performant to use CSS and HTML (or SVG?) to generate avatars. I have a core package (used to generate the initials), and if used that with html, it would be far better.
I use the service for our internal products, as gravatar fallbacks
Having said that, please pardon my ignorance, but:
What's the advantage over simply not using an avatar at all?