How can I make the header image fit correctly?

Hello,
I have a header image that I thought was just right until I looked at my site on a different computer and on an iPad. It's getting cut off strangely on both sides. I originally added white space on both sides of the image to try to correct it but that didn't work right. Then I tried using the "Header Image Alignment" option but because of the white space I added that's not right either.

I guess I don't understand how to make the image responsive like the theme is so it could scale needed.

Thanks for any suggestions you may have for me.
Laura