使用php为图像添加水印

function watermarkpic($filename) {

ini_set('max_input_time',300);

require 'config.php';  

$watermark = imagecreatefrompng('watermarknew.png');
$watermarkwidth = imagesx($watermark);
$watermarkheight = imagesy($watermark);

if(preg_match('/[.](jpg)$/',$filename)) {  
        $originalimage = imagecreatefromjpeg($path_to_image_directory . $filename);  
    } else if (preg_match('/[.](gif)$/',$filename)) {  
        $originalimage = imagecreatefromgif($path_to_image_directory . $filename);  
    } else if (preg_match('/[.](png)$/',$filename)) {  
        $originalimage = imagecreatefrompng($path_to_image_directory . $filename);  
    }  

$originalwidth = imagesx($originalimage);
$originalheight = imagesy($originalimage);

$maxsize = 800;
$imgratio = $originalwidth / $originalheight;

if($imgratio > 1) {
    $finalwidth = $maxsize;
    $finalheight = $maxsize / $imgratio;
}
else {
    $finalheight = $maxsize;
    $finalwidth = $maxsize * $imgratio;
}   

$finalimage = imagecreatetruecolor($finalwidth,$finalheight);

imagecopyresampled($finalimage,$originalimage,$finalwidth,$finalheight,$originalwidth,$originalheight);

imagecopymerge($finalimage,$watermark,$watermarkwidth,$watermarkheight,100);

//now move the file where it needs to go
if(!file_exists($path_to_medimage_directory)) {  
        if(!mkdir($path_to_medimage_directory)) {  
                die("There was a problem. Please try again!");  
        }  
     } 

 imagejpeg($finalimage,$path_to_medimage_directory . $filename);   
}

问题是水印具有透明背景,但它在图像上显示为黑色背景.我见过关于alpha混合等的东西,但我真的不知道这是什么.我想了解我在做什么,以及解决问题,以便水印是透明的.真实的画面应该填补空间.

提前致谢.