<?php

$url='../images'; //图片地址，只可以用./you/images这样的路径

$files=array();
if ($handle=opendir("$url")) {
    while(false !== ($file = readdir($handle))) {
          if ($file != "." && $file != "..") {
          if(substr($file,-3)=='gif' || substr($file,-3)=='jpg') $files[count($files)] = $file;
          }
    }
}
closedir($handle);

$random=rand(0,count($files)-1);
if(substr($files[$random],-3)=='gif') header("Content-type: image/gif");
elseif(substr($files[$random],-3)=='jpg') header("Content-type: image/jpeg");
readfile("$url/$files[$random]");