In the Customize section of your campaign, you can easily add a background that will apply to all parts of the offer. Just click on the tab labeled BGROUND. Here you can choose from adjustable patterns, backdrops, an online photo library, or upload your own background image.
When using a Backdrop, or Image, you will also see the “Gears” icon which allows for extra customization:
In the Image & Design Specs article there is not any mention of the background size for uploaded images, and for good reason. Different devices can skew the image to stretch bigger or shrink smaller, and so there is not a recommended size. The only limit to the size will be the file size, which has a max of 1 MB. Using an image with a pattern is a good option, which tends to work when stretched, tiled, or centered.