In the MasterPage.master page add the below code into the
<head></head>
region
<link href="Style.css" rel="stylesheet" type="text/css" /><pre lang="CSS">
Then create a .Css file with the name "Style.css" and drop it into the folder where the masterpage is.
In the Style.css add the below line
body
{
background-image: url('image.jpg');
}
If you do have any doubts please let us know.