Help designing loading a background graphic in designer

New designer here trying to build a new controls scheme for my Hubitat/Homeseer/Logitech systems.
I am interested in creating a background graphic to base my pages on but I cannot seem find any information as to how to import this into Designer. I see others doing it.
I see background images in the Images folder but I am unsure how to interact them or open one to use as a design template.
Is there any instructions or tutorials I can view to get my feet on the ground creating my design in Photoshop or Powerpoint? and how do I “assign” it to a project?
Sorry to be such a newbee struggling with what is probably a simple thing.

You Are The Artist! And Below Are Your Tools…

You can add your designs to the images folder. All images (files) use the underscore nomenclature otherwise the software will not present them if you use dashes