Multiple Author Signatures

I run a multi-author site and I wanted each author to have their own signature image display under each post.

When I enter this code in the functions.php it works perfectly, but obviously it puts the one image under ALL the posts…but it works.

add_filter('the_content','add_signature');
function add_signature($text) {
global $post;
if($post->post_type == 'post')
$text .= '<div class="signature">
<div style="text-align:center;">
<img src="http://booksbeyondtheend.com/katsig.png" alt="kat" />
</div>';
return $text;
}

So when I go with this code to separate it out for the multiple authors, I get a 500 error and the site is stating that the post part of the 4th line is causing the error. I don’t understand why though.

add_filter('the_content','add_signature’);
function add_signature($text) {
global $post;
if($post->post_type == 'post') {
          if ( get_the_author() == 'Kat MacKenzie' ) {
               $text .= '<div class="signature">
               <img src="http://booksbeyondtheend.com/katsig.png" alt="kat" />
               </div>';
          } else if ( get_the_author() == 'Alex Black' ) {
               $text .= '<div class="signature">
               <img src="http://booksbeyondtheend.com/alexsig.png" alt="alex" />
               </div>';
          } else if ( get_the_author() == 'Adeleas Wellington' ) {
               $text .= '<div class="signature">
               <img src="http://booksbeyondtheend.com/adeleassig.png" alt="adeleas" />
               </div>’;
          }

     return $text;
}

I don’t know how to write code so speak to someone with limited understanding of all that. Thanks!!
(also, I am using a child theme, twenty twelve theme fyi)