Post thumbnails are pretty useful and pretty easy to use in WordPress. Simply add:

add_theme_support('post-thumbnails'); 

To a theme's functions.php file and you'll get a Featured Image module on the admin screen for posts which allows you to select one.

It is also very easy to output that image as an HTML <img>:

the_post_thumbnail();

But what if you just need the URL? Say, you're going to use it as a background-image on an element rather than a content image. Unfortunately there is no super easy/obvious function for that.

Within the loop, you'll have to do:

$thumb_id = get_post_thumbnail_id();
$thumb_url_array = wp_get_attachment_image_src($thumb_id, 'thumbnail-size', true);
$thumb_url = $thumb_url_array[0];

Then $thumb_url will be that URL.