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.
If you've got a portrait book, you can choose to change the left and right pages individually. If you're making a landscape or square book, changes will affect the whole background.
You can change the background colour, and since we released version 3.4, you can now add a selection of themed background images.
Want to add your own background image?
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, tap on the Inspector and tap the Move to Back icon (the one on the left) to make sure everything else is layered on top of this image.
From the blog: