<?php

$imageTag = <<< LOB
<img border="0" src="/images/image.jpg" alt="Image" width="100" height="100" />
<img border="0" src="/images/not_match_image.jpg" alt="Image" width="100" height="100" />
LOB;

preg_match('%<img.*?src=["\'](.*?)["\'].*?/>%i', $imageTag , $result);
$foo = $result[1];
echo $foo;