首先下载：

composer require topthink/think-image

使用：

use think\Image;

 public function upload(){
        // 水印图片路径
        $waterMarkPath = 'watermark.png';
        $file = request()->file('file');
        $savename = \think\facade\Filesystem::disk('public')->putFile( 'pic', $file);
        if($savename){
            $return['path'] = '/upload/'.$savename;
            $image_path = 'upload/'.$savename;
            $return['status'] = 1;

            // 创建Image对象
            $image = Image::open($image_path);
            // 添加水印，参数分别是水印图片路径、位置、透明度
            $image->water($waterMarkPath, Image::WATER_SOUTHEAST, 50);
            // 保存处理后的图片
            $image->save($image_path);
        }else{
            // 上传失败获取错误信息
            $return['error']   = 1;
            $return['success'] = 0;
            $return['message'] = '上传出错'.$file->getError();
        }
        exit(json_encode($return));
    }