Automatically Set the Featured Image in WordPress

<?php
function autoset_featured() {
	global $post;
	$already_has_thumb = has_post_thumbnail($post->ID);
	if (!$already_has_thumb) {
		$attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );
		if ($attached_image) {
			foreach ($attached_image as $attachment_id => $attachment) {
				set_post_thumbnail($post->ID, $attachment_id);
			}
		}
	}
}
add_action('the_post', 'autoset_featured');
add_action('save_post', 'autoset_featured');
add_action('draft_to_publish', 'autoset_featured');
add_action('new_to_publish', 'autoset_featured');
add_action('pending_to_publish', 'autoset_featured');
add_action('future_to_publish', 'autoset_featured');
?>

Via Downgraf - How to Automatically Set the Featured Image in WordPress