GD Newbie Question

[VikR]

This is my first time using GD. I've got an image with a transparent background. All the other pixels are the identical color (#8bc4ed). I want to replace all those pixels with a given color, say #CCC5AC. Here's my code that doesn't work yet:

 

$filepath = $_SERVER['DOCUMENT_ROOT'] . "/images/source_image.gif"; 
$image = imagecreatefromgif($filepath); 

$indexToSearchFor = imagecolorat($image, 3, 0); 
imagecolorset($image, $indexToSearchFor, 204, 197, 172); 

$filepath = $_SERVER['DOCUMENT_ROOT'] . "/images/target.gif"; 
imagegif ($image,  $filepath);

 

The output image is identical to the input image - no color is replaced.

 

What is the correct way to do this?

 

Here is a link to the image I'm trying to change in case you can tell anything from it: http://www.generalwebapps.com/images/header_2_selected.gif.

 

Thanks very much to all in advance for any info!