think\Validate::image PHP Method

    protected function image($file, $rule)
    {
        if (!$file instanceof File) {
            return false;
        }
        $rule = explode(',', $rule);
        list($width, $height, $type) = getimagesize($file->getRealPath());
        if (isset($rule[2])) {
            $imageType = strtolower($rule[2]);
            if ('jpeg' == $imageType) {
                $imageType = 'jpg';
            }
            if (image_type_to_extension($type, false) != $imageType) {
                return false;
            }
        }
        list($w, $h) = $rule;
        return $w == $width && $h == $height;
    }