http://phpsnips.com/54/Trasparent-Gif#.VBAXI_mSySo

transparent_gif.php

<?php 
// $imageDirectory = the directory of the image with out the trailing slash 
// $thumbDirectory = the directory where you want to save the thumb 
// $imageName = the file name of the image example: myimage.gif 
// $thumbWidth = the final size of the thumb 
// $percent = the scaling size of the image. values = .1 - 1 

// This function will Save the image on your server 
function transparentGif($imageDirectory,$thumbDirectory, $imageName, $thumbWidth){ 
    $image = imagecreatefromgif("$imageDirectory/$imageName"); 
    $details = getimagesize("$imageDirectory/$imageName"); 
    $thumbHeight = $details[1] * ($thumbWidth / $details[0]); 
    $resized = imagecreatetruecolor($thumbWidth, $thumbHeight); 
    $colorTransparent = imagecolortransparent($image); 
    imagepalettecopy($image, $resized); 
    imagefill($resized, 0, 0, $colorTransparent); 
    imagecolortransparent($resized, $colorTransparent); 
    imagecopyresized($resized, $image, 0, 0, 0, 0, $thumbWidth, $thumbHeight, $details[0], $details[1]); 
    imagegif($resized,"$thumbDirectory/$imageName"); 
} 

// This function will only display the transparent gif, and not save it 
function transparentGifShow($imageDirectory,$thumbDirectory, $imageName,$percent = 1){ 
    $image = imagecreatefromgif("$imageDirectory/$imageName"); 
    $details = getimagesize("$imageDirectory/$imageName"); 
    $thumbHeight = $details[1] * $percent; 
    $thumbWidth = $details[0] * $percent; 
    $resized = imagecreatetruecolor($thumbWidth, $thumbHeight); 
    $colorTransparent = imagecolortransparent($image); 
    imagepalettecopy($image, $resized); 
    imagefill($resized, 0, 0, $colorTransparent); 
    imagecolortransparent($resized, $colorTransparent); 
    imagecopyresized($resized, $image, 0, 0, 0, 0, $thumbWidth, $thumbHeight, $details[0], $details[1]); 
    imagegif($resized); 
} 
?>