Try this:
private static System.Drawing.Image GetImg(string url)
{
System.Drawing.Image tmpimg = null;
var httpWebRequest = (HttpWebRequest)HttpWebRequest.Create(url);
var httpWebReponse = (HttpWebResponse)httpWebRequest.GetResponse();
Stream stream = httpWebReponse.GetResponseStream();
tmpimg = System.Drawing.Image.FromStream(stream);
return tmpimg;
}
And put this line in your click event handler:
GetImg("http://www.barcodes4.me/barcode/qr/myfilename.png?value=My%20QR%20Code").Save(Server.MapPath("~/Images") + "/barcode.png");