Header Graphics

I am still unable to crop the header image and when not cropped it will not appear WITHIN the header area, but BELOW the header, which is not what I want.

If I am to use MEDIA LIBRARY to adjust header images to their required size, please tell me what this size is, so that I can rescale and crop images within the Media Library.

I also do not really understand why it is said in the Header section that best the image should be 0 pixels high. But an image with 0 pixels height cannot be seen!