How do you get header Image(s) to automatically resize to the screen resolution?

Hello, I am trying to get the header image(s) on my blog to automatically resize according to the user's browser size or screen resolution. I plan to add more images in my rotation in the future as well. I am a total newbie so I'm not sure if this matters. Please help. Thank you.
My blog is http://adiama.com/ancestralconnections