Don't do this with regular expressions. Instead, parse the HTML. Take a look at Parse HTML With PHP And DOM. This is a standard feature in PHP 5.2.x (and probably earlier). Basically the logic for getting images is roughly:
$dom = new domDocument;
$dom->loadHTML($html);
$dom->preserveWhiteSpace = false;
$images = $dom->getElementsByTagName('img');
foreach ($images as $image) {
echo $image->getAttribute('src');
}
This should be trivial to adapt to finding images.