Divisor

Hi guys......I'm just recently starting to learn HTML and CSS and it's going great but I can't figure something out......I have a div like so.....<div Id="banner"><img src="my picture" width="970" height="160" alt="banner" /></div>my desire is to have the banner centre'd on page which I did accomplish but when I resize the browser window to a width smaller than that of the width specified in the CSS file for the div, everything appears to "hug" the right side of the screen ignoring the right margin.....I was looking for a way to have everything fixed so browser window resizing has no effect on the contents of the div whatever it might be......