Found some plugins that may (or not) match your requirements. Using this query string in the repository.
But, there’s one (WP-SnapAvatar) that although doesn’t address the particulars, consists in a little and very interesting function, that I’ll reproduce here:
/*
Plugin Name: WP-SnapAvatar
Version: 1.0
Plugin URI: http://darkx-studios.com/
Description: This plugin puts the commentor's website screenshot where there is no gravatar.
Author: Neacsu Alexandru
Author URI: http://darkx-studios.com
Copyright (c) 2008
Released under the GPL license
http://www.gnu.org/licenses/gpl.txt
*/
function site_avatar_hook($baseCode) {
$AutorUrl = get_comment_author_url();
$PictureLink = "http://images.websnapr.com/?url=$AutorUrl";
preg_match('@http://[^"\']+["\']@',$baseCode,$newSrc);
$newSrc = $newSrc[0];
if(trim($AutorUrl)){
$httpSrc = substr($newSrc,0,strlen($newSrc) - 1) . "&nocatch=" . rand(10,100) ."&default=" . $PictureLink;
$newAvatar=preg_replace('@src=(["\'])http://[^"\']+["\']@',"src="" . $httpSrc . """,$baseCode);
} else {
$newAvatar = $baseCode;
}
echo $newAvatar;
}
add_filter('get_avatar','site_avatar_hook',5,4);
For what you ask, I think that a combination of WP-SnapAvatar and this other one (Link Indication) would do the job.
But, I won’t get into that otherwise I end up writing a new plugin 🙂