1.  
<?php
$dir = dir('.');
$images = array();

while(($entry = $dir->read()) !== false) {
  if(strpos($entry, '.png') !== false) {
    $images[] = $entry;
  }
}

$rand_image = $images[rand(0, count($images) - 1)];

header("Content-type: image/png");
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");
header("Cache-Control: no-store, no-cache, must-revalidate");
header("Cache-Control: post-check=0, pre-check=0", false);
header("Pragma: no-cache");

readfile($rand_image);
?>

2. 
<?
$shouldredirect = 0; // :(

// Date in the past
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");

// always modified
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");

// HTTP/1.1
header("Cache-Control: no-store, no-cache, must-revalidate");
header("Cache-Control: post-check=0, pre-check=0", false);

// HTTP/1.0
header("Pragma: no-cache");

$d = dir('.');
$files = array();
while (($entry = $d->read()) !== false)
{
        if (strlen($entry) <= 4) continue;
        $a = strtolower(substr($entry,-4));
        if ($a == '.gif' || $a == '.jpg' || $a == 'jpeg' || $a == '.png')
                $files[] = $entry;
}
$d->close();
sort($files);
if (!isset($_SERVER['HTTP_REFERER'])) $_SERVER['HTTP_REFERER'] = '';
$x = $_SERVER['REMOTE_ADDR'] . $_SERVER['HTTP_REFERER'] . date('i');
$a = 0;
$i = strlen($x);
while ($i--)
{
        $a += $i * ord(substr($x,$i,1));
}
srand($a);
$fn = $files[rand() % count($files)];

if ($shouldredirect)
{
  header("Location: http://" . $_SERVER['HTTP_HOST']
                        . dirname($_SERVER['PHP_SELF'])
                        . "/" . $fn);
}
else
{
        $a = strtolower(substr($fn,-4));
        if ($a == '.gif')
                header("Content-Type: image/gif");
        else if ($a == '.png')
                header("Content-Type: image/png");
        else
                header("Content-Type: image/jpeg");
        header("Content-Length: " . filesize($fn));
        readfile($fn);
}
?>

3. 
<?
// pick from *.png
$imgArray = array();

if ($dir = opendir('.'))
{
   while (false !== ($file = readdir($dir))) // read files from the directory
   {
      if ($file != "." && $file != ".." && $file != "index.php")
      {
         array_push($imgArray, $file); // add it onto the array of valid images
      }
   }
}

// select from the array
$pic = array_rand($imgArray);

// first, send the header to the client
header("Content-type: image/png");

// next, read in the image
$fh = fopen($imgArray[$pic], "r");
$fp = fread($fh, filesize($imgArray[$pic]));
fclose($fh);

// finally, send the image
print $fp;
?>