To place your book cover in image, you will double click on the book template. You can add your book cover using the lightning bolt icon.
You can click on the lighting bolt icon in the bottom left corner of any book template, in the top stripe for single click editing or along the left side in the editing tools.
Clicking tThe lightning bolt will bring up your existing covers and prompt you to choose your own book cover!
Here is a video on using Community Templates: