GD image Question

[apepelis]

here is some crappy code that I wrote.
It works, but there has to be a better way to do this.
I did it this way cause on a truecolor image, imagecolorstotal($im); doesn't work and returns a 0.
Anyone know a work around so I can use imagecolorset?
tia

$im_size = GetImageSize ($original); 
$imageWidth = $im_size[0]; 
$imageHeight = $im_size[1]; 
if (stristr($original, ".png") != "")
	$im = imagecreatefrompng ($original); 
else
	$im = ImageCreateFromJPEG($original); 

	for($x=0;$x < $imageWidth;$x++)
	{
		for($y=0;$y < $imageHeight;$y++)
		{
		 $rgb = ImageColorAt($im, $x, $y);
	   $rgb = imagecolorsforindex($im,$rgb);
	   $t = ($rgb["red"]+$rgb["green"]+$rgb["blue"])/3;
	   imagesetpixel ($im, $x, $y, ($t<<16)+($t<<8)+$t);
   	}
  }
if (stristr($original, ".png")  != "")
{
	header("Content-type: image/Png");
	ImagePNG($im,null); 
}
else
{
	header("Content-type: image/jpeg");
	Imagejpeg($im,null,60); 
}