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`