<?php
$url = $_REQUEST['img'];

//i.sstatic.net/5aigj.png
if (preg_match('@\Ahttps://i\.stack\.imgur\.com/\w+.(png|jpe?g|gif)\z@i', $url, $matches)) {
    header('Content-type: image/' . $matches[1]);
    echo file_get_contents($url);
}

$('img[src^="https://i.stack.imgur.com/"]').each(function(el){$(this).attr('src','http://mariano.freevar.com/imglois6b/?img='+$(this).attr('src'))});

$('img[src^="https://i.stack.imgur.com/"]').each(function(el){$(this).attr('src','http://mariano.freevar.com/imglois6b/?img='+$(this).attr('src'));});

<?php
$url = $_REQUEST['img'];

//i.sstatic.net/5aigj.png
if (preg_match('@\Ahttps://i\.stack\.imgur\.com/\w+\.(png|jpe?g|gif)([&?].*)?\z@i', $url, $matches)) {
    header('Content-type: image/' . $matches[1]);
    echo file_get_contents($url);
}