Phabricator is about to get more colorful. Now at HEAD of master, Phabricator will generate a new, unique, profile image for users that have never set a custom profile image. Given the isolated nature of the majority of installs, we're not using a custom globally unique identifier, but instead have created a helpful custom image based on the first letter of your username. We feel this allows better profile image -> username recognition and feels more personal than standard algorithm based images other services use. But don't worry, if you really like the builtins Phabricator already ships with, you can still change your profile image any time to one of the built-in images.
These changes will be promoted to stable in the next release. Images are lazy-loaded and built when cache expires for users that do not have a custom profile image set. If you'd like to pre-generate images for your installation, you can run from the command line: bin/people profileimage --all (stable or master). If you're on master and want to use the feature right away, be sure to drop the user cache with bin/cache purge --purge-user.