Glad that gave you what you were looking for.
On the header, what you would do is to upload the image to your media library, get the URL of that image and replace URL_OF_IMAGE between the quote marks with the URL of your new image in the CSS below. The existing header image is 180px in height, and the following would allow you to increase that to 250px. If you want it taller than that, we will have to make some adjustments.
background: url("URL_OF_IMAGE") no-repeat scroll center top rgba(0, 0, 0, 0);