Github uses identicons for anyone without a profile picture.identicons - github.blog
An identicon is basically a visual representation of a hash value (of user id, email, IP address). But since github's identicon algorithm is not in public domain, this is a simple C++ implementation which uses random values to generate an identicon.
These are 420x420 png images representing the 5x5 identicon.
To compile Identicon.cpp on Ubuntu:
g++ Identicon.cpp `pkg-config --cflags --libs opencv`