Peralelize image downloads across hostnames

Hi all,

I have a simple function (in function.php) that replaces the image hostname to speedup page loading:

function cdn_images() {
	$content = ob_get_contents();
	$pattern = '/src=[\\"\']?([^\\"\']?.*(png))[\\"\']?/i';
	preg_match_all( $pattern, $content, $matches );
	foreach($matches[1] as $val) {
		$test = str_replace('www.domain.com', 'images.domain.com', $val);
		echo $test.'<br />';
	}
}

When placed on footer this obviously works well giving me new image paths. What I would like to do is to replace these images on the fly via WP hook.

Anyone knows how to do this?

Thanks!