Most the time its smart not to use just 1 big image. Just make text in tables, so it will automatically adjust itself to the screens resolution. As for just an image, I wouldn't know what to use, something under 800 by 600 cuz theres some people out there that run that. But then again some run at 1600 by 1200, lol.
I use: 730 x 375 as a .jpg resolution, so it exactly fits in the screens. Shortens the .jpg when using below 1024 x 768 and expanding it when using above resolution 1024 x 768.