Only Display a Featured Image on First Post Page

by

Use the global $page, which holds the current page number.

// Somewhere near the top
if ( $GLOBALS['page'] === 1 && $image_data = get_post_thumbnail_id() ) {
    if ( $image_data = wp_get_attachment_image_src( $image_data,  'full' ) ) {
        if ( $image_data[1] >= 500 )
            the_post_thumbnail( 'top-post' );
    }
}

// Further down. $image_data still exists, no need to grab it again.
if ( $GLOBALS['page'] === 1 && $image_data && $image_data[1] < 500 )
    the_post_thumbnail();

You’ll see I also a did a little spring cleaning:

  1. Removed redundant second $image_data = ...
  2. Added check that there is indeed a thumbnail
  3. Added check that $image_data exists