There are a few different ways you can set a background for your scene. This tutorial will go over the 2 most common ways.
The easiest way is to use the built in Background function.
From the Top Tool Bar, select the “Scene Settings” option, and then choose "Background"
The bottom rectangle represents the current color for the background. By default the background is black. Tap the rectangle to open the color palate and choose a new background color.
You can even save colors for later use. Tap and hold the squares at the bottom to save a color in that spot.
If you want to set an image as your background, you can tap the large box at the top of the window. From here you can select any image you have imported. For more information on importing images see the videos and help documentation on “importing Images” or "Asset Library".
You can also change how the background image is placed in the screen. By default it is placed in the center. Pressing the “Default” button at the bottom will revert the background to its original black state.
The Alternative method to setting a background is to simply add an image into your scene.
You can create a new layer and place it below all existing layers. For more information on creating a new layer see the help documentation on Layers.
Reorder the layers so the background layer is below any existing layers
Try experimenting with the different options of adding a background to see which works best for you.