I think it's cause your ratio gets off when you use:
width:100%;
height:100%;
Try this instead:
width: 100%;
height: auto;
That way the ration doesn't mess up, if you want the background to not mess up, try this:
background: url("images/Background.png") no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
EDIT:
If you mean centering the image, absolute poitioning is the absolute size of the browser, the full screen. While relative is the current position of the brower. I would use relative for cross-device purposes.