<?php
//全匹配
$str='<img border="0" src="1.jpg" alt=""/><img border="0" src="a.jpg" alt=""/><img src="test.jpg" />';
$pattern='/<img.*?src=[\'|\"](\S*?(\.png|\.jpg|\.gif))[\'|\"].*?[\/]?>/i';
preg_match_all($pattern,$str,$match);
//var_dump($match[1]);
var_dump($match);

//单
preg_match('/<img.+src=\"?(.+\.(jpg|gif|bmp|bnp|png))\"?.+>/i', '<img src="test.jpg" />', $match);
echo $match[1];

?>