$str = '<img border="0" src=\'/images/image.jpg\' alt="Image" width="100" height="100"/>';
preg_match('/(src=["\'](.*?)["\'])/', $str, $match); //find src="X" or src='X'
$split = preg_split('/["\']/', $match[0]); // split by quotes
$src = $split[1]; // X between quotes
echo $src;
Other regexp's can be used to determine if the pulled src tag is a picture like so:
if(preg_match('/([jpg]{3}$)|([gif]{3}$)|([jpeg]{3}$)|([bmp]{3}$)|([png]{3}$)/', $src) == 1) {
//its an image
}