IIRC Gravatar has an option to generate a default image if you didn't upload your own avatar. Imagine that the the has was 89A3 then the image was a 2x2 square with colors 8, 9, A and 3; using some number2rgb function so it was green, yellow, white and red or something like that. I'm making up the details! And they used other shapes and more than 4 parts, but I hope my description is good enough. I found some nice discussion with the details of the real implementation in https://meta.stackexchange.com/questions/17443/how-is-the-de...

A few years ago it was popular to make Gravatar "clones". One generate weird robots instead of geometric shapes. Another generated aliens. ...

In Magicell I get "403 Forbidden", I'm in Argentina.