Basically, I'd like to maybe do one or two things:

1. Give a different default image for whatever your UID ends in, 0-9.
2. Give a selection of images to choose from in Edit Picture.

For defaults, something like:

• Change PhabricatorUser::getDefaultProfileImageURI() to be $user->getDefaultProfileImageURI(), since it now depends on which user we're asking about.
• Change callsites call it as $user->... (hopefully no weird/hard ones).
• For users with an ID, use $user->getID() % 10 or similar to select an image. For users without an ID (logged out users, omnipotent user) choose the default or some special image.
• Existing images will be cached. You can add a 2 or something to the array in PhabricatorUser->getProfileImageVersion() to break the cache.

For choosing, something like:

• In PhabricatorPeopleProfilePictureController, change this line:

$default_image = PhabricatorFile::loadBuiltin($viewer, 'profile.png');

...to pick an image based on $viewer instead (profile2.png, or whatever). This will give you a consistent default image (but not a choice between multiple images).

• If you want to add more default options, use PhabricatorFile::loadBuiltin() to load builtin files and then just dump them into $images:

$builtins = array('x.png', 'y.png');
foreach ($builtins as $builtin) {
  $file = PhabricatorFile::loadBuiltin($builtin);
  $images[$file->getPHID()] = array(
    'uri' => $file->getBestURI(),
    'tip' => pht('Builtin Image'),
  );
}

That should make them show up, I think.