PHP-Image - Using GD in PHP (Help!)

[refined]

I have an image, greyscale.

 

I want to convert everything that is *NOT* white to black.

 

How can I do this? Is there such a function that exists to convert a greyscale image to 100% black or 100% white ?

[Barand]

try

<?php
$im = imagecreatefromjpeg('gmc.jpg');


$white = imagecolorclosest($im, 255,255,255);
$black = imagecolorallocate($im, 0,0,0);
$sx = imagesx($im);
$sy = imagesy($im);
for ($x=0; $x<$sx; $x++)
{
    for ($y=0; $y<$sy; $y++)
    {
        $c = imagecolorat($im, $x, $y);
        if ($c != $white)
        {
            imagesetpixel($im, $x, $y, $black);
        }
    }
}
header("content-type:image/jpeg");
imagejpeg($im);
imagedestroy($im);
?>