I just made today a class for Processwire that automates the process quite a bit. It's really basic, I will improve it in the future.
https://github.com/carlitoselmago/opengraphPW
It's used like this:
<head>
<?php
include_once("./opengraphPW/opengraphPW.php");
$OG=new openGraph(
$page, // current PW page, required
$pages, // PW pages wire object, required
$config->urls->templates.'img/cover.jpg', // A general image (1200x628px recomended) for pages like a home page where there's no content image, optional
"Title of your website"); // A generic title if the home element is called something like "home", optional
?>
</head>