On the off-chance you wan to do a single large photo for the background of your site, I recommend the Background Manager Plug-in which you can activate on your Dashboard/Plugins page. You can set a background image using the plug-in and it will re-size the photo to fit the screen of the person visiting your site.
You can even have several images and either set it to load a random image for each visit or page view, or to fade in a new image like a slideshow.
It has lots of great options.
Again, if that’s not what you’re looking for, let me know.