1. Click the page itself (the frame of the page should be highlighted in orange).
2. Click the Background button located on the upper style toolbar.
3. Click Change under the image icon and select the desired image.
4) Choose the image that will be the background of the page (Can't find the image? Try uploading it: How to upload an image? ).
Need more help? Try:
How to change aspects of an individual page without affecting the master template?