How to make the header image larger than 1440X500

The link to my website is http://www.thiswouldntfitonapostcard.com.

Right now, under appearances>header, the landscape theme will only let me resize my header to 500px in height and 1440px in width or smaller. I want my header to be 1500px in height and 2000px in width. How do I do this?

Also, because the header image will not cover the full width of the screen the sides of the image will be black automatically. How do I make the sides of the image transparent like the rest of the content area (wood, in this case)?

Any help would be greatly appreciated!! Thanks!