修改Anchor CMS中自定义字段的URL

So, I've recently set up a CDN for my website and have been going through and amending all of the URLs.

I need to amend every occurrence of http://helloarchie.blue/ to http://helloarchie.helloarchie.netdna-cdn.com/.

So far so good, except there's one thing I'm stumped on.

I have a custom field set up in the CMS for post thumbnails (the thumbnails that display on the home page). I upload these via the CMS and it automatically puts them in helloarchie.blue/content/ and this is the URL it uses to display them on the live site, and I can't figure out how to change it.

Here's the code for the thumbnail:

<img class="front-tn" src="<?php echo article_custom_field('thumbnail', 'http://helloarchie.helloarchie.netdna-cdn.com/img/template.jpg'); ?>" />

I've also found this in one of the PHP files which looks like it may have something to do with it:

if($value) {
        $html .= '<a href="' . asset('content/' . $value) . '" target="_blank">' . $value . '</a>';
    }

Can I add the URL in there somewhere for it to work?