Well, assuming you know CSS, you could just do something like this:
html,
body {
width: 100%;
height: 100%;
background-image: url('/path/or/url/to/your/image/here/');
background-size: auto;
}
Untested, and hastily scrapped out in the middle of class, but it ought to work. This would go either in a <style></style> section in your <head> section in your MOTD file, or in your stylesheet if you've already got one linked.