Newbie - create image error URGENT

Hi everyone! I'm still learning PHP and MYSQL using windows98, MYSQL, PHP 4 in my local hard drive. I want to learn uploading image and resizing image in my local machine. when I execute this code: <?PHP //setup image $height = 200; $width = 200; $im = ImageCreate($width,$height); $white = ImageCreateAllocate ($im, 255, 255, 255); $black = ImageCreateAllocate ($im, 0, 0, 0); //Draw on image ImageFill($im, 0, 0, $black); ImageLine($im, 0, 0, $width, $height, $white); ImageString($im, 4, 50, 150, "Sales", $white); //output image Header ("Content-type: image/png"); ImagePng ($im); //Clean up ImageDestroy($im); I get error message: "Fatal error: Call to undefined function: imagecreate() in D:\graphic_practise\index.php on line 15" What did I do wrong? any advice/ help is greatly appriciated. Thanks, Ardani