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