creating an image with php code for tracking and a/b testing

hey i am trying to create 300x250 image that will call a php script every time

someone is open it , i am getting traffic from few publishers for my site

but i am not sure they give me the correct statistic

i already tried to create an image pixel 300 x 250 but i cannot download it

 <?php

  // Create an image, 1x1 pixel in size
  $im=imagecreate(1,1);

   // Set the background colour
  $white=imagecolorallocate($im,255,255,255);

  // Allocate the background colour
 imagesetpixel($im,1,1,$white);

// Set the image type
 header("content-type:image/jpg");

 // Create a JPEG file from the image
 imagejpeg($im);

// Free memory associated with the image
imagedestroy($im);

     ?>

how can i make such an image?