Current Moodle code, used to invoke Gravatar API

return new moodle_url("http://www.gravatar.com/avatar/{$md5}", array('s' => $size, 'd' => $defaulturl->out(false)));

Gravatar's suggestion for the 'd=' parameter:
(https://en.gravatar.com/site/implement/images/)

1. 404: do not load any image if none is associated with the email hash, instead return an HTTP 404 (File Not Found) response
2. mm: (mystery-man) a simple, cartoon-style silhouetted outline of a person (does not vary by email hash)
3. identicon: a geometric pattern based on an email hash
4. monsterid: a generated 'monster' with different colors, faces, etc
5. wavatar: generated faces with differing features and backgrounds
6. retro: awesome generated, 8-bit arcade-style pixelated faces

suggested fix:

return new moodle_url("http://www.gravatar.com/avatar/{$md5}", array('s' => $size,'d' => 'identicon'));