How to Custom post thumbnails

Adding this PHP code to your wordpress theme will allow you to define a post thumbnail. Create a custom field called post-img and add a URL to your image.
<div style="background-image:url('.get_the_post_thumbnail_url($idd, 'large' ).')" class="single_slider">

<?php
         $postimageurl = get_post_meta($post->ID, 'post-img', true);
     if ($postimageurl) {
?>

<a href="<?php the_permalink(); ?>" rel="bookmark"><img src="<?php echo $postimageurl; ?>" alt="<?php the_title(); ?>" width="210" height="210" /></a>

<?php } else { ?>

<a href="<?php the_permalink(); ?>" rel="bookmark"><img src="<?php bloginfo('template_url'); ?>/images/wpbeginner.gif" alt="<?php the_title(); ?>" width="210" height="210" /></a>

<?php } ?>