Hi!
Unfortunately this plugin didn't help me because I couldn't customize the output in any way.
So I was reading about custom fields like crazy, tried a dozen of possibilities and... my site stopped working :)
Luckily I had a backup and couple of hours after that and a few another crashdowns I've finally found a sollution.
A very simple one :O
at the begining of index.php you just have to put the loop code and then for example
$something = get_post_meta($post->ID, 'Name of the custom field', $single = true);
close the loop
and then in any part of index.php you want it's enough to write:
<?php if($something !== '') { ?>
<a href="<?php echo get_post_meta($post->ID, 'Name of the custom field', true); ?>"><img class="url_img" src="/wp-content/picture.gif"/></a>
<?php } ?>
The if is necessary if you don't want to have this button all over your site :)
The a href gives you already the output in a form of hyperlink so in the custom field you just write an url(!) and the img clas allows you to customize the button however you want via CSS.
Repeat in single.php
Voila! :)