How to download image using WebClient.UploadString method?

I want to get image by sending json data to API. I use WebClient to download image.

        string URI = "http://my_api.com";
        string myParameters = "{'my': 'json_object'}";

        using (WebClient wc = new WebClient())
        {
            wc.Headers[HttpRequestHeader.ContentType] = "application/x-www-form-urlencoded";
            string HtmlResult = wc.UploadString(URI, myParameters);
        }

UploadString method's return type is string. How can I convert this result to ContentType: image/jpeg ?