when looking for a picture for the header, make sure you start thinking wide and short. when you take a picture landscapes are good, closeups are bad (not easy to trim). One suggestion I give is take a peice of paper and cut a one inch by eight inch rectangle out of it. Now hold this a 1 1/2 away from your eyes and look at what you can see. This is what your header will look like, but the photo will be the 8 1/2 X 11 that the paper takes up. you only get to use a small piece.
next you have to determine how wide you want your webpage to be. Because of the bigger monitors, I've picked 1440px as my width and have a height of 250 (you can take a look at zenofmud.org - haven't updated it much lately, it's mainly a family blog)
Since you have a MAC, I suggest getting EASYCROP (http://www.yellowmug.com/
) which makes cropping images a snap (sorry any windows folks reading this) Just drop an image on the left, move the crop bar around and then drag the image in the right window, to the desktop and it's cropped.
Then FTP it to the site