Image processing using PHP

hai,
I am using the code :
<?php
Header("Content-type: image/jpeg");
$image = ImageCreate(200,150);
$black = ImageColorAllocate($image,204,204,204);
$blue = ImageColorAllocate($image,0,0,255);
ImageLine($image,10,10,150,30,$blue);
ImageJPEG($image);
ImageDestroy($image);
?>

to generate and image and i am getting problems in displaying the
same.when execute this script i don't get either error or image.is the
code correct or should i add something to support this.
i am using openBSD m/c and php is running on this.Some book suggested me
abt GD library.how do i check whether that is present in the machine.any
other alternative for this.
pls help in this matter.
hoping for a positive reply
Arun