First you need to have uploaded the image you want to use to your image library. You do this using the Image button on the toolbar, and if you only want to use the image as a background, you'll need to delete it by right clicking it when it gets added to your page. You need to remember its exact filename eg., image.jpg.

Click Page Info on the toolbar, and in the background image field type images/ followed by the exact file name. You need to remember that the system will repeat the background image across and down from the top left corner of your screen.

You can try this now using one of the images below. Right click on the one you want, save it to your computer, and go through the process as described above. You'll need to rename the image before uploading as these are hosted on flickr.