- preg_match: documentation ( source)
<?php
$string = '"res":"https://my.site.com/image/I/fj23l6j2lgk_AM1200_.jpg"';
echo preg_match('~(?<=^"res":")https://my\.site\.com/image/I/[a-zA-z\d]{11}_\w*_\.jpg(?="$)~', $string, $out) ? $out[0] : 'no match';