function getRandomImage($dir,$type="random")
{
global $errors,$seed;

  if (is_dir($dir)) { 

  $fd = opendir($dir); 
  $images = array();

      while (($part = @readdir($fd)) == true) { 

          if ( eregi("(gif|jpg|png|jpeg|bmp)$",$part) ) {
              $images[] = $part;
          }
      }

    // adding this in case you want to return the image array
    if ($type == 'all') return $images;

    if ($seed !== true) {
      mt_srand ((double) microtime() * 1000000);
      $seed = true;
    }
     
      $key = mt_rand (0,sizeof($images)-1);

    return $dir . $images[$key];

  } else {
      $errors[] = $dir.' is not a directory';
      return false;
  }
}

$dir="./where the/images/are/";
getRandomImage($dir);

<?
$dir = "images/";
$i = 0;
if ($handle = opendir($dir)) {
  while (false !==($file[$i] = readdir($handle))){
    $i++;
  }
}
$img = $dir . $file[rand(2,$i - 1)];
echo "<img src=$img>";
?>