Pixelation with GD Resize?!?

[neodur]

I am using this resize class in a project:

<?
##############################################
# Shiege Iseng Resize Class
# 11 March 2003
# shiegege@yahoo.com
# http://kentung.f2o.org/scripts/thumbnail/
################
# Thanks to :
# Dian Suryandari <dianhau@yahoo.com>
/*############################################
Sample :
$thumb=new thumbnail("./shiegege.jpg");			// generate image_file, set filename to resize
$thumb->size_width(100);				// set width for thumbnail, or
$thumb->size_height(300);				// set height for thumbnail, or
$thumb->size_auto(200);					// set the biggest width or height for thumbnail
$thumb->jpeg_quality(75);				// [OPTIONAL] set quality for jpeg only (0 - 100) (worst - best), default = 75
$thumb->show();						// show your thumbnail
$thumb->save("./huhu.jpg");				// save your thumbnail to file
----------------------------------------------
Note :
- GD must Enabled
- Autodetect file extension (.jpg/jpeg, .png, .gif, .wbmp)
  but some server can't generate .gif / .wbmp file types
- If your GD not support 'ImageCreateTrueColor' function,
  change one line from 'ImageCreateTrueColor' to 'ImageCreate'
  (the position in 'show' and 'save' function)
*/############################################


class thumbnail
{
	var $img;

	function thumbnail($imgfile)
	{
		//detect image format
		$this->img["format"]=ereg_replace(".*\.(.*)$","\\1",$imgfile);
		$this->img["format"]=strtoupper($this->img["format"]);
		if ($this->img["format"]=="JPG" || $this->img["format"]=="JPEG") {
			//JPEG
			$this->img["format"]="JPEG";
			$this->img["src"] = ImageCreateFromJPEG ($imgfile);
		} elseif ($this->img["format"]=="PNG") {
			//PNG
			$this->img["format"]="PNG";
			$this->img["src"] = ImageCreateFromPNG ($imgfile);
		} elseif ($this->img["format"]=="GIF") {
			//GIF
			$this->img["format"]="GIF";
			$this->img["src"] = ImageCreateFromGIF ($imgfile);
		} elseif ($this->img["format"]=="WBMP") {
			//WBMP
			$this->img["format"]="WBMP";
			$this->img["src"] = ImageCreateFromWBMP ($imgfile);
		} else {
			//DEFAULT
			echo "Not Supported File";
			exit();
		}
		@$this->img["lebar"] = imagesx($this->img["src"]);
		@$this->img["tinggi"] = imagesy($this->img["src"]);
		//default quality jpeg
		$this->img["quality"]=90;
	}

	function size_height($size=100)
	{
		//height
    	$this->img["tinggi_thumb"]=$size;
    	@$this->img["lebar_thumb"] = ($this->img["tinggi_thumb"]/$this->img["tinggi"])*$this->img["lebar"];
	}

	function size_width($size=100)
	{
		//width
		$this->img["lebar_thumb"]=$size;
    	@$this->img["tinggi_thumb"] = ($this->img["lebar_thumb"]/$this->img["lebar"])*$this->img["tinggi"];
	}

	function size_auto($size=100)
	{
		//size
		if ($this->img["lebar"]>=$this->img["tinggi"]) {
    		$this->img["lebar_thumb"]=$size;
    		@$this->img["tinggi_thumb"] = ($this->img["lebar_thumb"]/$this->img["lebar"])*$this->img["tinggi"];
		} else {
	    	$this->img["tinggi_thumb"]=$size;
    		@$this->img["lebar_thumb"] = ($this->img["tinggi_thumb"]/$this->img["tinggi"])*$this->img["lebar"];
 		}
	}

	function jpeg_quality($quality=90)
	{
		//jpeg quality
		$this->img["quality"]=$quality;
	}

	function show()
	{
		//show thumb
		@Header("Content-Type: image/".$this->img["format"]);

		/* change ImageCreateTrueColor to ImageCreate if your GD not supported ImageCreateTrueColor function*/
		$this->img["des"] = ImageCreateTrueColor($this->img["lebar_thumb"],$this->img["tinggi_thumb"]);
    		@imagecopyresized ($this->img["des"], $this->img["src"], 0, 0, 0, 0, $this->img["lebar_thumb"], $this->img["tinggi_thumb"], $this->img["lebar"], $this->img["tinggi"]);

		if ($this->img["format"]=="JPG" || $this->img["format"]=="JPEG") {
			//JPEG
			imageJPEG($this->img["des"],"",$this->img["quality"]);
		} elseif ($this->img["format"]=="PNG") {
			//PNG
			imagePNG($this->img["des"]);
		} elseif ($this->img["format"]=="GIF") {
			//GIF
			imageGIF($this->img["des"]);
		} elseif ($this->img["format"]=="WBMP") {
			//WBMP
			imageWBMP($this->img["des"]);
		}
	}

	function save($save="")
	{
		//save thumb
		if (empty($save)) $save=strtolower("./thumb.".$this->img["format"]);
		/* change ImageCreateTrueColor to ImageCreate if your GD not supported ImageCreateTrueColor function*/
		$this->img["des"] = ImageCreateTrueColor($this->img["lebar_thumb"],$this->img["tinggi_thumb"]);
    		@imagecopyresized ($this->img["des"], $this->img["src"], 0, 0, 0, 0, $this->img["lebar_thumb"], $this->img["tinggi_thumb"], $this->img["lebar"], $this->img["tinggi"]);

		if ($this->img["format"]=="JPG" || $this->img["format"]=="JPEG") {
			//JPEG
			imageJPEG($this->img["des"],"$save",$this->img["quality"]);
		} elseif ($this->img["format"]=="PNG") {
			//PNG
			imagePNG($this->img["des"],"$save");
		} elseif ($this->img["format"]=="GIF") {
			//GIF
			imageGIF($this->img["des"],"$save");
		} elseif ($this->img["format"]=="WBMP") {
			//WBMP
			imageWBMP($this->img["des"],"$save");
		}
	}
}
?>

This is a rather common class being used for resizing images on the fly and also for uploaded images. My problem is that this class has already being extensively used and I don't want to replace it. But it's producing visibly pixelated images even at JPEG Quality set to 90.

Can someone check it thru and tell me if there's some problem in there that can be fixed to get better quality resized images. I a fairly recent GD 2.0 built on the server.