Glad to help ..
About the line ... In css one can "squeeze" many rules into one if the their properties have a similarity .
This means that the following :
background-color:#555; /*#redredgreengreenblueblue or #redgreenblue or rgb(0,145,255) */
background-repeat:repeat-x; /* repeat-y no-repeat */
background-position:<from left> <from top>; /*ems percents points pixels cm in etc. or a combination of top bottom center right left */
background-attachment:scroll; /* or fixed */
can turn into this
background:#555 url("image.jpg") repeat-x 100px 0;
so specifically the position of background image was defined 0 pixels from top and 50% well .. from left meaning that it will position it in the center .. although im sure that it can be more well explained but this far my knowledge lies.In that way when you resize the window the image will still be in the center as well as the rest of the content (because of text-align center and margin :0 auto) .
Hope i helped.. but you can still find on all information on css and html at the w3c .
Web design is fun .. so .. Have fun . (: