Since you only one it on one Page/Tab you would want to do it as a skin and then set the body { background-image: url(...) } tag to do it.  As for module containers you wouldn't necessarily need to turn them off to see through them if they were designed to have transparent backgrounds although that might look trashy.  You could easily duplicate the existing skin and create a new skin file with it's own CSS file.  Lets say you were using DNN-Blue for example.  Open up the folder for DNN-Blue on your server ([PortalRoot]/Portals/_default/Skins/DNN-Blue) and then create a copy of the .ascx file that you would want to duplicate.  Rename the copy to something you could remember (lets say I used Horizontal Menu - Fixed Width.ascx; I might rename it to Horizontal Fixed BG Image.ascx).  Now create a CSS file with the exact same name but with .css extension instead of .ascx.  In there you would only need some CSS for body { background... } and then now you can use that skin on the single page you were talking about.

One caveat with having the skin use body { background... } is that if this skin were also used as the "Admin" page skin then that background would also show up on things like the Text/HTML editor and the File/Link manager (when you click "Browse" server).  This looks really bad when you would otherwise expect to have a container with a solid background show over the background image.  There are workarounds for this but that is another topic for another day since that isn't your intention here.

Good luck!