Add additional class to get_the_post_thumbnail()?

I’m having a heck of a time in adding a class to the get_the_post_thumbnail() function. As you can see, in the codex page on it, I *should* be able to define an array and add a new class, but not only is it not working, but when I add it, it removes the output completely.

I have an if/then statement – so “if” something is one way, I need a particular class added, “else” if it’s another way, I need a different class added. I’ve tried several methods to do it, but nothing is working. (I even tried writing my own function to add_filter to it – but no go.)

My current code is like so:

// images
     $images = get_post_meta($post->ID, 'images', true);
     $image = $images['imgrep'];
     $title = $images['title'];
     $featured = get_the_post_thumbnail($post->ID, 'medium');

     // title representation for top left side of posts
     if(!empty($title)) {
       $title = '<img class="titlerep wp-post-image" src="' . $title . '" alt="" />' . "\n";
       $inuse == 'no';
     } else if(!empty($featured)) {  // if no title rep set, use the featured image
       $title = $featured;
       $inuse == 'yes';
     } else {  // if no title rep set, and no featured image, use the post title text
       $title = '<h2>' . get_the_title() . '</h2>' . "\n";
       $inuse == 'no';
     }

     // image representation for right side
     if(!empty($image)) {
       $image = '<img class="imgrep wp-post-image" src="' . $image . '" alt="" />' . "\n";
     } else if(!empty($featured) && $inuse == 'no') {  // if no title rep set, use the featured image
       $image = $featured;
     }

So I’ve been trying to simply change the $featured variable at the top, and instead of leaving it there, pulling it into the conditionals and adding the class. ($title should have ‘titlerep’ added, and $image should have ‘imagerep’ added). I’ve tried just adding “'class=titlerep'” to the end, writing an array of values where “'class' => 'titleprep'” is in there and added to the end,even tried popping in a small function for the apply_filter thing – and like I said, nothing is working. Anyone have any ideas on how to do this?

Thanks!