Hook bbcode_img_match and add the following code

bbcode.php

<?php
//* Do NOT include the opening php tag above. Copy the code shown below.

$urlbits = parse_url($link);

$salt = 'XXXX';
        
if ($urlbits['scheme']!='https')
    $link = '/imgproxy.php?u='.urlencode($link).'&h='.md5($salt.$link.$salt);

$retval = ($fullsize ? '<div class="size_fullsize">' : '')  . '<img src="' .  $link . '" border="0" alt="" />' . ($fullsize ? '</div>' : '');