This is div class="content", it has a fixed width of 700px
and it is centered with margin:0 auto;
Images are inline elements by default so they can be positioned with text-align values.
The image above resides within a "p" tag with text-align:center;
This is div class="image-box", it has a fixed width of 640px
and it is also centered with margin:0 auto;
The image above has display:block assigned to it. Since it is a block
element now it can be given a width and centered with margin:0 auto;