If a place a jpg image in a page and set height attribute to set pixels and width=100% will that make image stretch and fit window with browser resizing?

Why would you do that ??? I mean.. if you stretch an image above and beyond it's original size, it's going to get REALLY ugly looking when you start stretching it. It's really not a good idea to try to resize images that way.

I already have a background image using css. This image creates a horizontal pattern.
It can be done though right?
It won't look bad at all.

Why not just use the repeat and let it fill the screen, rather than stretching it ?

It doesn't "tile" well, but it does stretch well. If that can make sense.