I want to put an image on my page that I would like to go all the way across, however some are visiting in 800with screens and some are visiting with higher resolution. Is it possilbe somehow to make it so that if someone wisits with 800 with, one image (shorter one) shows up, for those with larger screens another image (longer one) shows up?
Yes its possible if you put the image in a swf, and just have the .swf show up, it will then automatically resize itself to fit the browser area, although this will cause pixelation and distortion :\. Unless of course it's a really high resolution picture, with a high DPI, but then it would still look distorted, maybe not as much as a small image. But if you have something vector inside the swf, then quality will not be lost.
You can do it but as FireFuze said you wil get pixelation, if its just colors and no image you can set it as your background image and test it in all sizes to see how it looks. But if its a image you really cannot do it without paying a price with the pixels and distortion.
You should just make a 1 pixel wide image in a background table, and it will tile across the page. Just make it as tall as the header, you can probably just slice the end 1 pixel of the header graphic and repeat it across the page.