Featured image in post with caption

Hi, first of all thanks for great roots;)

So, I want to add caption to featured image in my posts. It’s not a thumbnail.

I’ve modified init.php like this…

set_post_thumbnail_size(150, 150, true);
add_image_size('category-thumb', 750, 300, true);

…and for displaying it I’m using this one:

<?php the_post_thumbnail('category-thumb', array('class' => 'aligncenter'))?>

I saw previous topic in here about caption, but adding <figcaption> and ->post_excerpt do nothing.

I’ve followed those topics around the web, but I can only show thumbnail and not my 'category-thumb'.