<?php
  $bg = array('bg-01.jpg', 'bg-02.jpg', 'bg-03.jpg', 'bg-04.jpg', 'bg-05.jpg', 'bg-06.jpg', 'bg-07.jpg' ); // array of filenames

  $i = rand(0, count($bg)-1); // generate random number size of the array
  $selectedBg = "$bg[$i]"; // set variable equal to which random filename was chosen
?>

<style type="text/css">
<!--
body{
background: url(images/<?php echo $selectedBg; ?>) no-repeat;
}
-->
</style>

Set up and array of filenames, which correspond to the file names of the images you are trying to randomize.

Randomize Background Image

php, background

PHP