thumbnail creation with GD2

I am trying to manipulate an uploaded image from a form. The system is PHP 4.3.3 and GD2 2.0.15 compatible reports on phpinfo().

I am following the PHP manual by using GD2 but I get the following errors:

Warning: imagesx(): supplied argument is not a valid Image resource in c:\inetpub\wwwroot\sixfive.co.uk\upload.php on line 24 Warning: imagesx(): supplied argument is not a valid Image resource in c:\inetpub\wwwroot\sixfive.co.uk\upload.php on line 25 Warning: imagesy(): supplied argument is not a valid Image resource in c:\inetpub\wwwroot\sixfive.co.uk\upload.php on line 26 Warning: imagecopyresampled(): supplied argument is not a valid Image resource in c:\inetpub\wwwroot\sixfive.co.uk\upload.php on line 40 Warning: imagedestroy(): supplied argument is not a valid Image resource in c:\inetpub\wwwroot\sixfive.co.uk\upload.php on line 47

Here is my code:

<?
include("accesscontrol.php");

$title="Administration";
session_start();
include("header.txt");

      $uploadDir = "C:/inetpub/wwwroot/domain.com/images/";
      $fileName = $_FILES['file']['tmp_name'];
      $userfileName = str_replace(" ","_",$_FILES['file']['name']);
    move_uploaded_file($fileName, $uploadDir. $userfileName);

      $source = imagecreatefromjpeg($uploadDir. $userfileName);
      $dest = imagecreate(100, 100);

    $system=explode(".",$userfileName);
    if (preg_match("/jpg|jpeg/",$system[1])){
        $src_img=imagecreatefromjpeg($uploadDir. $userfileName);
            echo("jpg");
    }
    if (preg_match("/png/",$system[1])){
        $src_img=imagecreatefrompng($uploadDir. $userfileName);
    }

      $original_x=imageSX($src_img);
    $original_y=imageSY($src_img);
    if ($original_x > $original_y) {
        $thumb_w=100;
        $thumb_h=$original_y*(100/$original_x);
    }
    if ($original_x < $original_y) {
        $thumb_w=$original_x*(100/$original_y);
        $thumb_h=100;
    }
    if ($original_x == $original_y) {
        $thumb_w=100;
        $thumb_h=100;
    }
    $dst_img=ImageCreateTrueColor($thumb_w,$thumb_h);
    imagecopyresampled($dst_img,$src_img,0,0,0,0,$thumb_w,$thumb_h,$original_x,$original_y);
      if (preg_match("/png/",$system[1])){
        imagepng($dst_img,$uploadDir."th_".$userfileName);
    } else {
        imagejpeg($dst_img,$uploadDir."th_".$userfileName);
    }
    imagedestroy($dst_img);
    imagedestroy($src_img);

    $sql = "Insert into Image (ImageName,ImageDescription,uploaddate,imagefile,thumbnailfile,viewcount,AlbumId) VALUES ('".$HTTP_POST_VARS['name']."','".$HTTP_POST_VARS['description']."',now(),'".$userfileName."','th_".$userfileName."',0,".$HTTP_POST_VARS['galleryId'].")";
      dbDo(&$result,$sql);

include("footer.txt");      
?>

Can anyone help?