为啥上传图片中含有透明通道，图片就会变成黑色

<!--?php<br /--><br>class Images{<br><br>
var $inputName;                 //控件名<br>
var $allowType = array(<br>
'image/gif','image/jpg','image/jpeg','image/png','image/x-png','image/pjpeg'<br>
);
                           //上传类型<br>
var $allowSize = 1048576;
//限制大小<br>
var $saveDir = "";      //保存目录<br>
var $FileName = ""; //文件名<br>
var $isRename = false;               //是否重命名，默认为true<br>
var $errID = 0;                     //错误代码，默认为0<br>
var $errMsg = "";                   //错误信息<br>
var $savePath = "";                 //保存路径<br>
var $ImgSize = 0; //图片尺寸<br><br>
function __construct($inputName,$allowType="",$allowSize="",$saveDir="",$isRename=true){<br>
if(empty($inputName)){<br>
$this->chk_err(-1);       //无传入控件名<br>
}else{<br>
$this->inputName = $inputName;<br>
}<br><br>
if(!empty($allowType)) $this->allowType = $allowType;<br>
if(!empty($allowSize)) $this->allowSize = $allowSize;<br>
if(!empty($saveDir)) $this->saveDir = $saveDir;<br>
if(!empty($isRename)) $this->isRename = $isRename;<br>
}<br><br>
function is_uploaded(){<br>
if(empty($_FILES[$this->inputName]['name'])){<br>
$this->chk_err(4);    //没有文件被上传<br>
}else{<br>
if(is_uploaded_file($_FILES[$this->inputName]['tmp_name'])){<br>
return true;<br>
}else{<br>
$this->chk_err(-2);       //文件上传不合法<br>
}<br>
}<br>
}<br><br>
function chk_type(){<br>
if(!in_array($_FILES[$this->inputName]['type'],$this->allowType)){<br>
$this->chk_err(-3);         //上传的文件类型不被允许<br>
}else{<br>
return true;<br>
}<br>
}<br><br>
function chk_size(){<br>
if($_FILES[$this->inputName]['size'] > $this->allowSize){<br>
$this->chk_err(-4);          //上传的文件过大<br>
}else{<br>
return true;<br>
}<br>
}<br><br>
function move_uploaded(){        //移动上传文件<br>
if(!$this->is_uploaded()){<br>
return false;<br>
}<br><br>
if(!$this->chk_size()){<br>
return false;<br>
}<br><br>
if(!$this->chk_type()){<br>
return false;<br>
}<br><br>
//重命名<br>
if($this->isRename){<br>
$arrTmp = pathinfo($_FILES[$this->inputName]['name']);<br>
$extension = strtolower($arrTmp['extension']);<br>
$file_newname = $this->FileName; //重命名新文件<br>
}else{<br>
$file_newname = $_FILES[$this->inputName]['name'];<br>
}<br>
<br>
if(!file_exists($this->saveDir)){       //判断保存目录是否存在<br>
mkdir($this->saveDir,0777,true);    //建立保存目录<br>
}<br><br>
//移动文件<br>
$result = move_uploaded_file($_FILES[$this->inputName]['tmp_name'],$this->saveDir."/".$file_newname);<br><br>
if($result){<br>
$path = $this->savePath = $this->saveDir."/".$file_newname;
//文件的成功保存路径<br>
return $path;<br>
}else{<br>
$this->chk_err($_FILES[$this->inputName]['error']);<br>
}<br>
<br>
}<br><br>
//判断出错信息<br>
function chk_err($errID){<br>
$this->errID = $errID;<br><br>
switch($this->errID){<br>
case -4:<br>
$this->errMsg = '只能上传1M以内的图片 | [返回]';<br>
break;<br>
case -3:<br>
$this->errMsg = '这不是一张图片 | [返回]';<br>
break;<br>
case -2:<br>
$this->errMsg = '文件上传不合法 | [返回]';<br>
break;<br>
case -1:<br>
$this->errMsg = '无控件名传入 | [返回]';<br>
break;<br>
case 1:<br>
$this->errMsg = '上传的文件超出了php.ini中upload_max_filesize设定的最大值 | [返回]';<br>
break;<br>
case 2:<br>
$this->errMsg = '上传文件的大小超过了HTML表单中MAX_FILE_SIZE选项指定的值 | [返回]';<br>
break;<br>
case 3:<br>
$this->errMsg = '文件只有部分被上传 | [返回]';<br>
break;<br>
case 4:<br>
$this->errMsg = '请选择一张图片 | [返回]';<br>
break;<br>
default:<br>
break;<br>
}<br>
return false;<br>
<br>
}<br><br>
function get_errMsg(){