Implement "identicon", "wavatar", "monsterid" and "retro" sets
Gravatar allows clients to request that missing images be replaced with a random-looking pattern or character that is different for each hashed email address. They are called: identicon, wavatar, monsterid and retro.
This is useful for sites that want to ensure that not everybody has the same "default" icon but also that comments from the same people can be visually tied together (since they share the same image).
Our implementation should be entirely static and pre-generated. The basic idea would be to find a set of parameters (e.g. color of the line, position of the lines, shape or thickness of the lines) that can be varied and combined. Then we could use the first X hexdigits of the photo hash to set these parameters and combine them into a complete image.
The number of hexdigits we look at will determine how many images will be needed (i.e. how much disk space and how many variations are needed).
(Currently, we redirect to Gravatar when one of these default patterns is requested.)