Thread: Enable Featured Image support for WP 3.1 & custom post types

// activate theme support items
if (function_exists('add_theme_support')) { // added in 2.9

// this theme uses post thumbnails
add_theme_support('post-thumbnails', array('post', 'page', 'ad_listing'));
set_post_thumbnail_size(100, 100); // normal post thumbnails

// add default posts and comments RSS feed links to head
add_theme_support( 'automatic-feed-links' );
}