How to display an image (Image generation)

hai ,
I am using the code
<?php
Header("Content-type: image/gif");
$image = ImageCreate(100,50);
$black = ImageColorAllocate($image,0,0,0);
$white = ImageColorAllocate($image,255,255,255);
ImageArc($image,50,25,48,0,$white);
ImageGif($image);
Imagedestroy($image);
?>
to display an image but the code is not working. how shud the code be
and do i need to enable any libraries to do this. i have to get an image
and get the midpoint(xy coordinate) of that image . how do i do that.
if i run the above code i won't get any error but nothing will be
displayed. how do i do this.
Arun