2つのファイルと同じディレクトリにpicsというサブディレクトリを置き、そのpicsにjpegファイルを入れてあげれば良し。あとは、index.phpにアクセスするだけ。

index.php

<?php
$dh = new DirectoryIterator("pics");

$files = array();
foreach($dh as $file){
  if(preg_match("/[.]jpg$/", $file)) $files[] = "$file";
}
?>
<html>
<head>
<title>スライドショー</title>
<style>
body {background: black;}
# thumbnails {height: 140px; width: 100%; overflow: auto;}
# pic {text-align: center; height: 400px; padding: 20px;}
</style>
<script>
var image_list = [
<?php $first = true; foreach($files as $image){ ?>
<?php echo ($first ? "" : ", "); ?>"<?php echo ($image); ?>"
<?php $first = false; } ?>
];

var curimage = 0;

function switchimg(ind){
  var image = image_list[ind];
  var obj = document.getElementById("selimg");
  obj.src = "scale.php?image="+image+"&y=400";
  curimage = ind;
}

function nextimage(){
  curimage++;
  if(curimage >= image_list.length) curimage = 0;
  switchimg( curimage );
}

window.setInterval("nextimage()", 2000);
</script>
</head>
<body>
<div id="thumbnails">
<table width="100%">
<tr>
<?php $ind = 0; foreach($files as $image) {?>
<td width="160" nowrap align="center">
<a href="javascript:switchimg( <?php echo($ind); ?> )">
<img height="100" src="scale.php?image=<?php echo($image); ?>&y=100" border="0" />
</a>
</td>
<?php $ind++;} ?>
</tr>
</table>
</div>
<div id="pic">
<img id="selimg" height="400" src="scale.php?image=<?php echo($files[0]);
   ?>&y=400" />
</div>
</body>
</html>

scale.php

<?php
$image = $_GET["image"];
$maxy = $_GET["y"];

$im = imagecreatefromjpeg("pics/".$image);
$curx = imagesx($im);
$cury = imagesy($im);
$ratio = $maxy / $cury;
$newx = $curx * $ratio;
$newy = $cury * $ratio;
$oim = imagecreatetruecolor($newx, $newy);
imageantialias($oim, true);
imagecopyresized($oim, $im, 0, 0, 0, 0,
        $newx, $newy, $curx, $cury);
header("content-type: image/jpeg");
imagejpeg($oim);
?>