get_first_image.php

function get_first_image($html)
{
	// We're trying to find and retrieve the SRC of the first image tag in the content
	preg_match("/<img(.*)>/Uise", $html, $matches);

	if(@trim($matches[1]) !== "")
	{
		// Found an image tag!
		// Get the SRC parameter ...
		preg_match("/src=\"(.*)\"/Uise", $matches[1], $image);

		// ... and return it
		return $image[1];
	}
	else
	{
		// Return the path to an "image not found", or something similar
		return "...";
	}

}

Credit goes to Wogan May: http://php.woganmay.com/2010/04/01/get-the-first-image-from-a-wordpress-post/