It's a simple matter to change the background of your page.
Tap on the Inspector when you have nothing highlighted, and you'll see the Page Inspector (or, if you've already tapped the Inspector from another item, you'll see another tab for Page.
You can change the background colour, or choose from a selection of themed backgrounds.
Want to use your own image as the background?
You can add your own image by simply adding a photo. Check this guide for making sure you know what size image will fill a page.
For an added measure, when you've added the picture, open the Inspector to use the Move to Back icon (the one on the left) to make sure everything else is layered on top of this image. Then, lock the image in place so it won't accidentally move.