You’re using a get function, which does not echo by default. Update the PHP portion to the following:
<?php echo wp_get_attachment_image($image, 'category-large-thumb' ); ?>
Also, wp_get_attachment_image() returns a full image object – so I don’t think that’s what your after.
It also most seems like you’d just want the_attachment_link:
<?php the_attachment_link( $id, $fullsize, $deprecated, $permalink ); ?>
So – it’d turn into:
<?php the_attachment_link(get_post_meta($post->ID, 'image_medium', true)); ?>