Open /admin/sources/base/core.php. Find:
else if( ipsRegistry::$settings['allow_gravatars'] )
{
/* Try a gravatar, if all else fails */
$av_hash = md5( $member['email'] );
$s = $lowestSize ? "&s={$lowestSize}" : '';
$blank_av = urlencode(ipsRegistry::$settings['avatars_url'] . '/blank_avatar.gif' );
return "<img src='http://www.gravatar.com/avatar/{$av_hash}?d={$blank_av}{$s}' alt='' />";
}return "<img src='" . ipsRegistry::$settings['avatars_url'] . '/blank_avatar.gif' . "' alt='' />";