PHP PNG照片上传，保持透明度

我完全被这个问题难住了。。。我需要一个照片上传脚本，保持png文件的透明度。目前，透明区域为黑色。非常感谢您的帮助。这是我的代码：

    if(isset($_POST)) {
    require_once('siteInfo.php');
    session_start();
    $ThumbSquareSize        = 90;
    $BigImageMaxSize        = $_POST['size'];
    $ThumbPrefix            = "thumb_";
    $DestinationDirectory   = '../Uploads/';
    $Quality                = 100;
    if (!isset($_FILES['ImageFile']) || !is_uploaded_file($_FILES['ImageFile']['tmp_name'])) {
        die('Please select a file.'); 
    }
    $RandomNumber   = rand(0, 9999999999); 
    $ImageName      = str_replace(' ','-',strtolower($_FILES['ImageFile']['name'])); 
    $ImageSize      = $_FILES['ImageFile']['size']; 
    $TempSrc        = $_FILES['ImageFile']['tmp_name'];
    $ImageType      = $_FILES['ImageFile']['type']; 
    switch (strtolower($ImageType)) {
        case 'image/png':
            $CreatedImage =  imagecreatefrompng($_FILES['ImageFile']['tmp_name']);
            break;
        case 'image/gif':
            $CreatedImage =  imagecreatefromgif($_FILES['ImageFile']['tmp_name']);
            break;          
        case 'image/jpeg':
        case 'image/pjpeg':
            $CreatedImage = imagecreatefromjpeg($_FILES['ImageFile']['tmp_name']);
            break;
        default:
          //  die('Unsupported file format. Please upload a PNG, GIF, or JPG file.'); //output error and exit
    }
    list($CurWidth,$CurHeight)=getimagesize($TempSrc);
    $ImageExt = substr($ImageName, strrpos($ImageName, '.'));
    $ImageExt = str_replace('.','',$ImageExt);
    $ImageName      = preg_replace("/''.[^.''s]{3,4}$/", "", $ImageName); 
    $NewImageName = $ImageName.'-'.$RandomNumber.'.'.$ImageExt;
    $thumb_DestRandImageName    = $DestinationDirectory.$ThumbPrefix.$NewImageName; //Thumb name
    $DestRandImageName          = $DestinationDirectory.$NewImageName; //Name for Big Image
    if (resizeImage($CurWidth,$CurHeight,$BigImageMaxSize,$DestRandImageName,$CreatedImage,$Quality,$ImageType)) {
        if(!cropImage($CurWidth,$CurHeight,$ThumbSquareSize,$thumb_DestRandImageName,$CreatedImage,$Quality,$ImageType)) {
            echo 'Error Creating thumbnail';
        }
        $query = "Update Content set ".$_POST['field']."='".$NewImageName."' WHERE ID=".$_POST['ID'];
        mysqli_query($dbc, $query);
        mysqli_close($dbc);
        echo 'Uploaded file: <span>'.$NewImageName.'</span>.';
    } else {
        die('Resize Error');
    }
}
function resizeImage($CurWidth,$CurHeight,$MaxSize,$DestFolder,$SrcImage,$Quality,$ImageType) {
    if ($CurWidth <= 0 || $CurHeight <= 0) {
        return false;
    }
    $ImageScale         = min($MaxSize/$CurWidth, $MaxSize/$CurHeight); 
    $NewWidth           = ceil($ImageScale*$CurWidth);
    $NewHeight          = ceil($ImageScale*$CurHeight);
    $NewCanves          = imagecreatetruecolor($NewWidth, $NewHeight);
    if (imagecopyresampled($NewCanves, $SrcImage,0, 0, 0, 0, $NewWidth, $NewHeight, $CurWidth, $CurHeight)) {
        switch(strtolower($ImageType)) {
            case 'image/png':
                imagepng($NewCanves,$DestFolder);
                break;
            case 'image/gif':
                imagegif($NewCanves,$DestFolder);
                break;          
            case 'image/jpeg':
            case 'image/pjpeg':
                imagejpeg($NewCanves,$DestFolder,$Quality);
                break;
            default:
                return false;
        }
        if (is_resource($NewCanves)) { imagedestroy($NewCanves); }  
        return true;
    }
}
function cropImage($CurWidth,$CurHeight,$iSize,$DestFolder,$SrcImage,$Quality,$ImageType){    
    if ($CurWidth <= 0 || $CurHeight <= 0) {
        return false;
    }
    if ($CurWidth>$CurHeight) {
        $y_offset = 0;
        $x_offset = ($CurWidth - $CurHeight) / 2;
        $square_size    = $CurWidth - ($x_offset * 2);
    } else {
        $x_offset = 0;
        $y_offset = ($CurHeight - $CurWidth) / 2;
        $square_size = $CurHeight - ($y_offset * 2);
    }
    $NewCanves  = imagecreatetruecolor($iSize, $iSize); 
    if (imagecopyresampled($NewCanves, $SrcImage,0, 0, $x_offset, $y_offset, $iSize, $iSize, $square_size, $square_size)) {
        switch(strtolower($ImageType)) {
            case 'image/png':
                imagepng($NewCanves,$DestFolder);
                break;
            case 'image/gif':
                imagegif($NewCanves,$DestFolder);
                break;          
            case 'image/jpeg':
            case 'image/pjpeg':
                imagejpeg($NewCanves,$DestFolder,$Quality);
                break;
            default:
                return false;
        }
        if (is_resource($NewCanves)) { imagedestroy($NewCanves); } 
        return true;
    }
}