Snippet Caching Problem in IE

/**
 * Gallery Snippet
 * ________________________________________
 * 
 *
 * @author Roland Schär
 * @date   2006-02-19
 */

$basefolder = '/assets/gallery/';
$maxwidth = isset($maxsize) ? $maxsize :  120;
$imgRoot = $_SERVER['DOCUMENT_ROOT'] . $basefolder;



if( isset($_GET['image']) && isset($_GET ['gallery']) ) {
	$images = getImages($imgRoot . $_GET['gallery']);
	$pos = array_search($_GET['image'],$images);

	$overview .= '<p><a href="'.$_SERVER['SCRIPT_NAME'].'?id='.$modx->documentIdentifier.'&gallery='.$_GET['gallery'].'"><< zurück zur Galerie</a></p><div>';
	if($pos>0)
			$overview .= '<p style="float:left;width:50%;text-align:left;"><a href="'.$_SERVER['SCRIPT_NAME'].'?id='.$modx->documentIdentifier.'&gallery='.$_GET['gallery'].'&image='.$images[$pos-1].'">vorheriges Bild</a></p>';
	if($pos<count($images)-1) 
			$overview .= '<p style="float:right; width:50%;text-align:right;"><a href="'.$_SERVER['SCRIPT_NAME'].'?id='.$modx->documentIdentifier.'&gallery='.$_GET['gallery'].'&image='.$images[$pos+1].'">nächstes Bild</a></p>';
	$overview .= '</div>';
	$overview .= '<div class="galleryImageLarge"><img src="assets/gallery/thumb.php?pic=' . $_GET['gallery'] . '/' . $_GET['image']  . '&width=450" alt="" /></div>';

}else if(isset($_GET['gallery'])){
	
	$overview = '<p><a href="'.$_SERVER['SCRIPT_NAME'].'?id='.$modx->documentIdentifier.'"><< zurück zur Übersicht</a></p>';
	$images = getImages($imgRoot . $_GET['gallery']);
	for($i=0; $i<count($images); $i++){
		$overview .= '<div class="galleryImg"><a href="'.$_SERVER['SCRIPT_NAME'].'?id='.$modx->documentIdentifier.'&gallery='.$_GET['gallery'].'&image='.$images[$i].'"><img src="assets/gallery/thumb.php?pic=' . $_GET['gallery'] . '/' . $images[$i]  . '&width='.$maxwidth.'" border="0" alt="" /></a></div>';
	}

}else{

	$galleries = getGalleries($imgRoot);
	for($i=0; $i<count($galleries); $i++){
		$previewImage = getPreviewImage($imgRoot . $galleries[$i]);
		if($previewImage != '') {
			$img = '<img src="assets/gallery/thumb.php?pic=' . $galleries[$i] . '/' . $previewImage . '&width=120" alt=""  border="0" />';
		}else{
			$img = '';
		}
		$overview .= '<div class="gallery"><a href="'.$_SERVER['SCRIPT_NAME'].'?id='.$modx->documentIdentifier.'&gallery='.$galleries[$i].'">' . $img . $galleries[$i].'</a></div>';
	}

}

return $overview;

/** **/
function getGalleries($fld){
	$arrTopics = array();
	$i = 0;
	$dir = opendir($fld);
	while( $file = readdir($dir) ){
		if( $file != "." && $file != ".." ){
			if( is_dir($fld."/".$file) ){
				$arrTopics[$i] = $file;
				$i++;
			}
		}
	}
	return $arrTopics;
}

function getImages($fld){
	if( file_exists($fld) ){
		$imgPath = opendir( $fld );
		$i = 0;
		while($imgFile = readdir($imgPath)){
			if( is_file($fld."/".$imgFile) ){
				$arrFiles[$i] = $imgFile;
				$i++;
			}
		}
	}
	return $arrFiles;
}

function getPreviewImage($fld){
	$arrFiles = array();
	if( file_exists($fld) ){
		$imgPath = opendir( $fld );
		$i = 0;
		while($imgFile = readdir($imgPath)){
			if( is_file($fld."/".$imgFile) ){
				$arrFiles[$i] = $imgFile;
				$i++;
			}
		}
	}
	if( !in_array('titelbild.jpg',$arrFiles) ){	
		return $arrFiles[0];
	}elseif( count($arrFiles) == 0 ){
		return '';
	}
	return 'titelbild.jpg';
}

<?php
header('Last-Modified: '.gmdate('D, d M Y H:i:s', $timestamp).' GMT'); 
header("Content-type: image/jpeg");
header('Content-transfer-encoding: binary');

$im=imagecreatefromjpeg($pic);
$orange=imagecolorallocate($im, 220, 210, 60);
$px=(imagesx($im) - 7.5 * strlen($string)) / 2;
$old_x=imageSX($im);
$old_y=imageSY($im);
$new_w=(int)($width);

if (($new_w<=0) or ($new_w>$old_x)) {
 $new_w=$old_x;
}
$new_h=($old_x*($new_w/$old_x));

if ($old_x > $old_y) {
   $thumb_w=$new_w;
   $thumb_h=$old_y*($new_h/$old_x);

}
if ($old_x < $old_y) {
   $thumb_w=$old_x*($new_w/$old_y);
   $thumb_h=$new_h;
}
if ($old_x == $old_y) {
   $thumb_w=$new_w;
   $thumb_h=$new_h;
}

$thumb=ImageCreateTrueColor($thumb_w,$thumb_h);
imagecopyresized($thumb,$im,0,0,0,0,$thumb_w,$thumb_h,$old_x,$old_y);
imagejpeg($thumb,"",90);
imagedestroy($thumb);
?>