use elgg_get_data_path() instead of $CONFIG->wwwroot
then create icon.php and return path to icon.php in identicon_usericon_hook along with required with and size then in icon.php have this code
$news_guid = get_input('guid');
$contents = readfile(filepath);
$size = strtolower(get_input('size'));
if (!in_array($size, array('large', 'medium', 'small', 'tiny', 'master', 'topbar')))
$size = "large";
header("Content-type: image/jpeg");
header('Expires: ' . gmdate('D, d M Y H:i:s \G\M\T', strtotime("+10 days")), true);
header("Pragma: public");
header("Cache-Control: public");
header("Content-Length: " . strlen($contents));
header("ETag: \"$etag\"");
echo $contents;