<link rel="image_src" href="http://www.example.com/image.jpg" />
Being that the case, I had two problems:
1. The image I wanted to show on Facebook was only used for the front page, so it couldn't be linked by Facebook whenever it crawled throughout the post; and,
2. The link tag had to be used from outside the loop.
Using this great post as an initial step, I came up with this. Feel free to use it if you find it useful:
<?php if (is_single()) { ?>
<?php
global $wp_query;
$postid = $wp_query->post->ID;
?>
<link rel="image_src" href="[Image-folder]/<?php echo get_post_meta($postid, 'Customfield', true); ?>" />
<?php } ?>
And there you go! Images from custom fields being used as you wish on Facebook Share.
