Thematic/Full Page Image

I’m building on Thematic, and I’d like to use a background image for my entire site; header, body, and footer. I’d like to utilize all of the screen real estate, and not just the real estate specified in the thematic dimensions.

Can anyone tell me:

a: what dimensions should I use in the creation of the image?
b: how do I go about implementing use of the image?

Also, I’d settle for being able to use 3 separate images for header, body, and footer as long as they used up all the screen real estate and not just that weird center area that gets inhabited when I use a banner in place of the blog title.

I hope I’m explaining this correctly. Thank you for reading.