WP Custom Fields

/****************************************************************
* To use custom fields put the following code where you want use
* Replace 'key' by your custom fields name
****************************************************************/
<?php echo get_post_meta($post->ID, 'key', true); ?>

/****************************************************************
* The following code will show custom fields when you set custom field
* Other wise it won't show the custom fields
* Replace 'key' by your custom fields name
****************************************************************/
<?php if ( get_post_meta( get_the_ID(), 'key', true ) ) : ?>
    <a href="<?php the_permalink() ?>" rel="bookmark">
            <img class="thumb" src="<?php echo get_post_meta( get_the_ID(), 'key', true ); ?>" alt="<?php the_title(); ?>" />
    </a>
<?php endif; ?>