[Skubent]

1. Сервер-сайд.
2.

<script>
images = new Array();
images[1] = 'test1.gif';
images[2] = 'test2.gif';
function placeimg()
{
  document.write('<img src="path_to_pics/' + images[Math.floor(Math.random() * 2 + 1)] + '.gif">');
}
</script>