wow. i can't believe i figured this out!
to make YOUR gravatar appear on the blog you're pinging, you need to do this:
1) open xmlrpc.php
2) find the following code:
$comment_post_ID = (int) $post_ID;
$comment_author = $title;
$this->escape($comment_author);
$comment_author_url = $pagelinkedfrom;
$comment_content = $context;
$this->escape($comment_content);
$comment_type = 'pingback';
3) under the line $comment_author = $title;, insert:
$comment_author_email = 'name@youremail.com';
obviously replace "name@youremail.com" with the eMail addressed you used to register your gravatar.
4) upload and replace the xmlrpc.php in the root of your blog directory.
that's it! Now when your blog sends a pingback, your gravatar will appear with the pingback!
thanks again haochi for reminding me of the $comment_author_email variable.