The problem you may be facing is relative paths to the image.
Depending on where your document is located relative to the image things may not work as expected.
In CSS you cannot use the tilde (~) symbol to get the project root, i tried and it does not work, so you have to use root based paths using /.
I have also seen people hop up the tree using ../../ notation, but thing this is to risky and better to work root down.
e.g.
<body style="background-image:url(/folder/folder/images/image.jpg)" runat="server">
if you miss the leading '/' you end up with a relative path.