Low bandwidth Windows wallpaper

Personally I wouldn't use wallpaper at all - I'd leave the desktop a solid color, and make them each a different color (essentially color-coding them). This is the lowest bandwidth method to do this.

An option would be to use BgInfo from Sysinternals to display the system information which would allow you to differentiate each session by simply seeing the machine name on the desktop. And you get the added bonus of seeing all the other system information as well.

It automatically displays relevant information about a Windows computer on the desktop's background, such as the computer name, IP address, service pack version, and more. You can edit any field as well as the font and background colors, and can place it in your startup folder so that it runs every boot, or even configure it to display as the background for the logon screen.

I would recommend to create a wallpaper with computer name, IP and whatever details you need. Also, you can use a very handy and free tool called BGInfor (http://technet.microsoft.com/en-us/sysinternals/bb897557.aspx) Good luck :)