BACKGROUND IMAGE - jumps around

i coded in a static background for my site in the stylesheet.css file in my template as follows:

body {
margin: 0px;
padding: 0px;
text-align: center;
background-image: url("http://mysite.com/image/backgroundtest.jpg");
background-position: 50% 50%;
background-repeat: no-repeat;
}

it looks fine on the "home page" but then depending on the other pages i click...such as a category or contact us, etc. the background will shift down slightly. why is this and how can i resolve the issue?

Is this a full sized background? and the syntax in your backgroun-image is not quite finished.

If not you need to add "repeat" at the end of the

background-image: url('../image/[nameofimage.jpg') repeat;

Hope this sorts the problem out for you.

Sorry meant to add this bit:

If you have a link to the pages it would help to see what you are describing.

is there a way to make it left and top aligned as well as the text locked to it so it lines up with the background. like this: http://tinyurl.com/2oh2eg
as youll see the header text "clothing, shoes & bags, etc" is locked with the background and still on the left and right actually locked. how is this done?

thanks!