To get a value of a single custom field use
<?php
$post_id = $post->ID;
$key = 'value'; // change value to what custom field you are looking for
// To retrieve only the first value of given key
$customfield = get_post_meta($post_id, $key, 'true');
// To use as default
$customfield = get_post_meta($post_id, $key);
// Then check what you got
if($customfield)
?>
<img class="thumb" src="https://wordpress.stackexchange.com/questions/25935/<?php echo get_post_meta($post_id, $key, true) ?>" alt="<?php the_title(); ?>" />
<?php
else
echo 'whatever you want to show by default';
?>
For further reading goto codex
Hope it helps you