If you set the container that wraps the side bar and main content at 960px width then that will be the width of the page - a fixed width. Set the container to - width: 90%; and a max-width: 960px; then it will grow no wider than 960px but will shrink in width as you narrow the browser window.
If you want the container horizontally centered add - margin: 0 auto; to the containers css selector.
Also I would get rid of the width and height set on the image so it looks like :
<img class="imgalice" src="alice.jpg" alt="Alice in Wonderland">
Then amend your css selector to as below:
.imgalice {
max-width: 100%;
height: auto;
}