Before we cover this, we'll first explain how to change the background image on any screen in your event,
How to use one of your own images?
Repeat the steps above, but instead of choosing an image from the Library, click the More images... button (at th bottom left corner of the Media Library window).
You can now browse your computer to find an image that is not part of the Media Library.
You should try and ensure that any image you use is not an overly large file and ideally it should be the same pixel size as your computer display ( for example 1920 x 1080 pixels ).
More information can be found in this article.