You should try your code with a double slash at the end:
uploadto = ("ftp://" + severip + ":1919//" + "IMG/"+ username + ".png")
you should also try this approach:
uploadto = ("ftp://ftp." + severip + ":1919//" + "IMG/"+ username + ".png")
try to change your request method like this:
request.Method = WebRequestMethods.Ftp.DownloadFile;