先前条件：第一步安装compose，已安装忽略。
先上截图看效果

一、在fastadmin的根目录里面输入命令安装think-image

composer require topthink/think-image

二、找到公共上传类，application/common/library/Upload.php，在最下面新添加一个压缩方法

    /**
     * 图片压缩方法
     */
    public function compressImage(&$params, $ratio = 10)
    {
        //图片大于300K才进行压缩，先判断是横图，还是坚图，设置图片宽最大820，高最大560，做出相对应的比例压缩
        if ($params['filesize'] > 300000) {
            //横图＝宽比高大
            if ($params['imagewidth'] >= $params['imageheight']) {
                if ($params['imagewidth'] >= 820) {
                    $width = 820;
                    $height = round($params['imageheight'] / ($params['imagewidth'] / 820));
                } else {
                    $width = $params['imagewidth'];
                    $height = $params['imageheight'];
                }
            } else { //坚图＝高比宽大
                if ($params['imageheight'] >= 560) {
                    $height = 560;
                    $width = round($params['imagewidth'] / ($params['imageheight'] / 560));
                } else {
                    $width = $params['imagewidth'];
                    $height = $params['imageheight'];
                }
            }

            $pathname = $params['url'];
            $file = $this->file;
            $image = \think\Image::open($file);
            $image->thumb($width, $height)->save('.' . $pathname);
            $image_root_path = ROOT_PATH . '/public' . $pathname;
            $params['imagewidth'] = $width;
            $params['imageheight'] = $height;
            $params['filesize'] = filesize($image_root_path);
        }
    }

三、在application/common/library/Upload.php文件中，找到里面的 upload() 方法，在文件上传完成，将要添加到attachment表的图片数据时进行等比压缩处理

        // 开启压缩
        if (in_array($this->fileInfo['type'], ['image/gif', 'image/jpg', 'image/jpeg', 'image/bmp', 'image/png', 'image/webp']) || in_array($this->fileInfo['suffix'], ['gif', 'jpg', 'jpeg', 'bmp', 'png', 'webp'])) {
            $this->compressImage($params);
        }