If you don't want to use javascript for that you can use php which is better.
Here is the simpliest way that I could thing ok.
Must be a .PHP FileCODE
<?php
// linking to the images that you want to display.
// feel free to edit within the single quotes ''
$images = array(
'http://farm1.static.flickr.com/175/428230889_3b3994a0b9_m.jpg',
'http://farm2.static.flickr.com/1371/1008816651_22d46ee2d1_m.jpg',
'http://farm4.static.flickr.com/3121/2765637403_268188b4a7_m.jpg',
'http://farm4.static.flickr.com/3008/2611822886_ee757ee4ed_m.jpg',
'http://farm1.static.flickr.com/1/2998240_18286b2677_m.jpg'
);
// printing out the image.
echo "<img src=\"" . $images[array_rand($images)] . "\" alt=\"\" />";
?>
You can add as many images as you want.
Make sure you add the comma , after all except the last one.
Edit: I don't no if Piczo allow php.