NOTE: A lot of these backgrounds are border backgrounds (the bottom list). Since the guestbook is centered on the page, it normally looks fine, but the dividers cross over into the design slightly on a 800x600 screen and the messages themselves cross over on a 640x480 screen, however the message text box has a background, so it should be ok. If you feel that most of your visitors will be using a 640x480 screen resolution (everything looks bigger), then you will want to use the tiled backgrounds. Take a look here for our test guestbook which has a border background.
All you have to do is choose the background you like and then put the corresponding URL into the box that asks you for your background when you set up or modify your guestbook. An easy way to get the URL there is to copy and paste it. (easier than typing it all)
Windows Users: Click your mouse right in front of the URL and hold your mouse button down and drag it over the url until the whole address is highlighted. Then go up to the "edit" menu at the top of your screen and pull it down to "copy". Next, go back to the form where you need to enter the address and click in the box so that your cursor is placed there. Go up to the "edit" menu again and pull the mouse down to "paste". Voila! The URL appears in the box where it belongs.
You can also push the "ctrl" button and "c" at the same time to copy and "ctrl" and "v" at the same time to paste.
Mac Users: You can do the same as above except if you want to use the shortcut keys instead of the menu, you need to push the "apple" key and "c" to copy and the "apple" key and "v" to paste.
Click on the URL to view the background.