Make sure your image data does not contain some header information at the beginning:
imageCode = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAMgAAABkC...This will cause an error. Just remove everything in front of and including the first comma, and you good to go.
imageCode = "iVBORw0KGgoAAAANSUhEUgAAAMgAAABkC...Covert base64 image to a stream
Stream memStream = new MemoryStream(Convert.FromBase64String(base64StringData));