Displaying Meta Box Image

I don’t know exactly how that plugin works, and you have posted code that is probably missing context but you need to define $post_meta_data before running line:

$custom_image = $post_meta_data['image'][0];

It seems that the plugin is using post meta so you should need something like this:

<?php echo get_post_meta($post->ID, $prefix.'hjemmeside', true); ?>

But rewritten to save the data as a variable instead of echo it. Guessing based on the code you posted, I believe you need:

$post_meta_data = get_post_meta($post->ID, $prefix.'image', true);
if (!empty($post_meta_data[0])) {
  $custom_image =  wp_get_attachment_image($post_meta_data[0], 'thumbnail');
}
echo $custom_image; // if desired

Untested, but stands a good chance of working.