I want to put a background picture for the irst screen of my website, with a big "enter" button in the middle. Whenever I view the site on different monitors and resolutions, the image doesn't take up the full screen. How do I fix this?

There are a bunch of methods you could use to remedy this, some more practical than others (it would be easier to help you if you show us what you're trying to do).

1) Set up the page with a repeating background so that it fills the page and center the image on the page. *Recommended*
2) Oversize the image to be larger than the largest screen resolution.
3) Use Javascript to resize the image to the size of the window.
etc.

I recommend #1, as that's the easiest and the only one that's really practical.