il codie è questo:
<?php ini_set("memory_limit", "200000000"); // for large images so that we do not get "Allowed memory exhausted"?>
<?php
// upload the file
if ((isset($_POST["submitted_form"])) && ($_POST["submitted_form"] == "image_upload_form")) {

// file needs to be jpg,gif,bmp,png and 4 MB max
if (($_FILES["image_upload_box"]["type"] == "image/jpeg" || $_FILES["image_upload_box"]["type"] == "image/pjpeg" || $_FILES["image_upload_box"]["type"] == "image/gif" || $_FILES["image_upload_box"]["type"] == "image/png") && ($_FILES["image_upload_box"]["size"] < 4000000))
{


// some settings
$max_upload_width = 2592;
$max_upload_height = 1944;

// if user chosed properly then scale down the image according to user preferances
if(isset($_REQUEST['max_width_box']) and $_REQUEST['max_width_box']!='' and $_REQUEST['max_width_box']<=$max_upload_width){
$max_upload_width = $_REQUEST['max_width_box'];
}
if(isset($_REQUEST['max_height_box']) and $_REQUEST['max_height_box']!='' and $_REQUEST['max_height_box']<=$max_upload_height){
$max_upload_height = $_REQUEST['max_height_box'];
}


// if uploaded image was JPG/JPEG
if($_FILES["image_upload_box"]["type"] == "image/jpeg" || $_FILES["image_upload_box"]["type"] == "image/pjpeg"){
$image_source = imagecreatefromjpeg($_FILES["image_upload_box"]["tmp_name"]);
}
// if uploaded image was GIF
if($_FILES["image_upload_box"]["type"] == "image/gif"){
$image_source = imagecreatefromgif($_FILES["image_upload_box"]["tmp_name"]);
}
// BMP doesn't seem to be supported so remove it form above image type test (reject bmps)
// if uploaded image was BMP
if($_FILES["image_upload_box"]["type"] == "image/bmp"){
$image_source = imagecreatefromwbmp($_FILES["image_upload_box"]["tmp_name"]);
}
// if uploaded image was PNG
if($_FILES["image_upload_box"]["type"] == "image/png"){
$image_source = imagecreatefrompng($_FILES["image_upload_box"]["tmp_name"]);
}


$remote_file = "image_files/" . $_FILES["image_upload_box"]["name"];
$ext = substr($remote_file, strrpos($remote_file, '.'));
$remote_filename = substr($remote_file, 0, -strlen($ext));
$i = 1;
while (file_exists($remote_file)) {
$remote_file = $remote_filename . $i . $ext;
$i++;
}
imagejpeg($image_source,$remote_file,100);
chmod($remote_file,0644);



// get width and height of original image
list($image_width, $image_height) = getimagesize($remote_file);

if($image_width>$max_upload_width || $image_height >$max_upload_height){
$proportions = $image_width/$image_height;

if($image_width>$image_height){
$new_width = $max_upload_width;
$new_height = round($max_upload_width/$proportions);
}
else{
$new_height = $max_upload_height;
$new_width = round($max_upload_height*$proportions);
}


$new_image = imagecreatetruecolor($new_width , $new_height);
$image_source = imagecreatefromjpeg($remote_file);

imagecopyresampled($new_image, $image_source, 0, 0, 0, 0, $new_width, $new_height, $image_width, $image_height);
imagejpeg($new_image,$remote_file,100);

imagedestroy($new_image);
}

imagedestroy($image_source);


header("Location: index.php?upload_message=Immagine caricata con successo&upload_message_type=success&show_image=$r emote_file");
exit;
}
else{
header("Location: index.php?upload_message=Assicurati che l'immagine abbia estenzione jpg, gif o png e che la sua grandezza sia minore o uguale a 4MB&upload_message_type=error");
exit;
}
}
?>
Il form di upload è questo:

<form action="index.php" method="post" enctype="multipart/form-data" name="image_upload_form" id="image_upload_form" style="margin-bottom:0px;">
<label>Grandezza massima dei file 4MB. Si possono inserire file jpg, gif, png.</label>

<input name="image_upload_box" type="file" id="image_upload_box" size="40" />
<input type="submit" name="submit" value="Carica Immagine" />

e oltre al problema delle gif che non si muovono c'è quello delle png che non rimangono con lo sfondo trasparente.