Hi Caroline, There are 2 ways to do this. The simplest way is to use the Doodle effect which can be found towards the end of the Effects tab. You can use this to paint a flat color over the background. Adjust the tool’s brush size and hardness to use it most effectively.
The other way is using the Eraser tool. With this you can start with a background image, and then drag your main image on top from the Photo Basket (at the bottom of the editor). You then resize and erase areas from the top image using the Eraser Tool. You can watch a demo video of this here.