You can either define a color or upload an background image. Follow the instructions on top for format and size. If you choose a background image, make sure it works with the rest of the design (a white background would not be the best idea for example). Also, it might make sense to choose something calming to avoid confusing your guests.
Tip: There are a few templates that serve as an inspiration. You can also download those templates as a PowerPoint document and edit it. This way you have the right format right from the get-go.