you can make the background image any size you want.
the reason its only a thin strip is because its only a blue gradient, so to fill the screen the same image can be repeated horizontally insteading of loading one huge image, cuts down loading times doing it with a thin strip

in includes/templates/coolblue/css/stylesheet.css at line 11
body {
background: url("../images/background.jpg") repeat scroll 0 0 transparent;
color: #000000;
font-family: verdana,arial,helvetica,sans-serif;
font-size: 62.5%;
margin-top: 20px;
}
that controls the body tag and the background image, if you dont want to have the background image repeating, change this line:
background: url("../images/background.jpg") repeat scroll 0 0 transparent;
to this:
background: url("../images/background.jpg") no-repeat scroll 0 0 transparent;
this repeats horizontally only:
background: url("../images/background.jpg") repeat-x scroll 0 0 transparent;
and this repeats vertically only:
background: url("../images/background.jpg") repeat-y scroll 0 0 transparent;
choose which ever line you need from the above ^^
to change the background image, just save the new image as background.jpg and upload it to
includes/templates/coolblue/images
and overwrite the original,
when viewing your site for the first time after uploading the new image, make sure you give the screen a hard refresh (Ctrl and F5 at the same time)
