一、在kindeditor/php/upload_json.php中，文件末尾添加方法

//水印方法
function imageWaterMark($groundImage, $waterPos=0, $waterImage='', $alpha=80, $waterText='', $textFont=9, $textColor='#FF0000'){
    $isWaterImage = FALSE;
    $formatMsg = '不支持該圖片格式！請使用GIF、JPG、PNG格式的圖片。';
    //讀取水印文件
    if(!empty($waterImage) && file_exists($waterImage)){
        $isWaterImage = TRUE;
        $water_info = getimagesize($waterImage);
        $water_w = $water_info[0];//取得水印圖片的寬
        $water_h = $water_info[1];//取得水印圖片的高
        switch($water_info[2]){//取得水印圖片的格式
            case 1:$water_im = imagecreatefromgif($waterImage);break;
            case 2:$water_im = imagecreatefromjpeg($waterImage);break;
            case 3:$water_im = imagecreatefrompng($waterImage);break;
            default:die($formatMsg);
        }
    }
    //讀取背景圖片
    if(!empty($groundImage) && file_exists($groundImage)){
        $ground_info = getimagesize($groundImage);
        $ground_w = $ground_info[0];//取得背景圖片的寬
        $ground_h = $ground_info[1];//取得背景圖片的高
        switch($ground_info[2]){//取得背景圖片的格式
            case 1:$ground_im = imagecreatefromgif($groundImage);break;
            case 2:$ground_im = imagecreatefromjpeg($groundImage);break;
            case 3:$ground_im = imagecreatefrompng($groundImage);break;
            default:die($formatMsg);
        }
    }else{
        alert("水印圖片不存在！");
    }
    //水印位置
    if($isWaterImage){//圖片水印
        $w = $water_w;
        $h = $water_h;
        $label = "圖片的";
    }
    if(($ground_w<$w) || ($ground_h<$h)){
        echo "需要加水印的圖片的長度或?qū)挾缺人?.$label."還小，無法生成水印！";
        return; // www.zuimoban.com
    }
    switch($waterPos){
        case 0://隨機
            $posX = rand(0,($ground_w - $w));
            $posY = rand(0,($ground_h - $h));
            break;
        case 1://1為頂端居左
            $posX = 0;
            $posY = 0;
            break;
        case 2://2為頂端居中
            $posX = ($ground_w - $w) / 2;
            $posY = 0;
            break;
        case 3://3為頂端居右
            $posX = $ground_w - $w;
            $posY = 0;
            break;
        case 4://4為中部居左
            $posX = 0;
            $posY = ($ground_h - $h) / 2;
            break;
        case 5://5為中部居中
            $posX = ($ground_w - $w) / 2;
            $posY = ($ground_h - $h) / 2;
            break;
        case 6://6為中部居右
            $posX = $ground_w - $w;
            $posY = ($ground_h - $h) / 2;
            break;
        case 7://7為底端居左
            $posX = 0;
            $posY = $ground_h - $h;
            break;
        case 8://8為底端居中
            $posX = ($ground_w - $w) / 2;
            $posY = $ground_h - $h;
            break;
        case 9://9為底端居右
            $posX = $ground_w - $w;
            $posY = $ground_h - $h;
            if(!$isWaterImage){
                $posY = $ground_h - $h-20;
            }
            break;
        default://隨機
            $posX = rand(0,($ground_w - $w));
            $posY = rand(0,($ground_h - $h));
            break;
    }
    //設(shè)定圖像的混色模式
    imagealphablending($ground_im, true);
    if($isWaterImage){//圖片水印
        //imagecopy($ground_im, $water_im, $posX, $posY, 0, 0, $water_w,$water_h);//拷貝水印到目標文件
        //生成混合圖像 背景透明png圖片會串色
        //imagecopymerge($ground_im, $water_im, $posX, $posY, 0, 0, $water_w, $water_h, $alpha);
        //生成混合圖片 不會串色
        imagecopymerge_alpha($ground_im, $water_im, $posX, $posY, 0, 0, $water_w, $water_h, $alpha);
    } else {//文字水印
        if( !empty($textColor) && (strlen($textColor)==7)){
            $R = hexdec(substr($textColor,1,2));
            $G = hexdec(substr($textColor,3,2));
            $B = hexdec(substr($textColor,5));
        } else {
            die("水印文字顏色格式不正確！");
        }
        imagestring($ground_im, $textFont, $posX, $posY, $waterText, imagecolorallocate($ground_im, $R, $G, $B));
    }
    //生成水印后的圖片
    @unlink($groundImage);
    switch($ground_info[2]){//取得背景圖片的格式
        case 1:imagegif($ground_im,$groundImage);break;
        case 2:imagejpeg($ground_im,$groundImage,95);break;//注意這里的100，經(jīng)測試，100是圖片質(zhì)量最佳 的，但文件大小會增加很多，95的時候質(zhì)量還不錯，大小和原來的差不多。作者采用95的值。
        case 3:imagepng($ground_im,$groundImage);break;
        default:die("上傳錯誤");
    }
    //釋放內(nèi)存
    if(isset($water_info)) unset($water_info);
    if(isset($water_im)) imagedestroy($water_im);
    unset($ground_info);
    imagedestroy($ground_im);
}
//生成背景透明水印圖片
function imagecopymerge_alpha($dst_im, $src_im, $dst_x, $dst_y, $src_x, $src_y, $src_w, $src_h, $pct){
    $opacity = $pct;
    // getting the watermark width
    $w = imagesx($src_im);
    // getting the watermark height
    $h = imagesy($src_im);

    // creating a cut resource
    $cut = imagecreatetruecolor($src_w, $src_h);
    // copying that section of the background to the cut
    imagecopy($cut, $dst_im, 0, 0, $dst_x, $dst_y, $src_w, $src_h);
    // inverting the opacity
//    $opacity = 100 - $opacity;

    // placing the watermark now
    imagecopy($cut, $src_im, 0, 0, $src_x, $src_y, $src_w, $src_h);
    imagecopymerge($dst_im, $cut, $dst_x, $dst_y, $src_x, $src_y, $src_w, $src_h, $opacity);
}

二、找到 $json = new Services_JSON(); 大約在129行，下面添加：

//水印添加方法
    $water_img = $_SERVER['DOCUMENT_ROOT'].'/image/watermark.png'; //水印圖片
    $water_mark = 1;//1為加水印, 其它為不加
    $water_pos = 9;//水印位置
    $water_alpha = 50;//水印透明度
    $water_text = '';//水印字符串,默認填寫空;
    //$water_fontfile = $_SERVER['DOCUMENT_ROOT'] .'/upfiles/fonts/arial.ttf';//文字水印使用的字體;
    if($water_mark == 1){
        imageWaterMark($file_path, $water_pos, $water_img, $water_alpha, $water_text);
    }

來源：
http://www.adminbuy.cn/ecshopjc/1147.html
http://blog.sina.com.cn/s/blog_884391e30100wsbh.html