If you would like to use one sheet of your diagram as the background for other diagram sheets, hover over the sheet that you want to set a background, select "...", select "Set background" and lastly select the sheet from the selection menu which you would like to use as the background sheet.

After the selection, it will appear as a static image background on your current sheet.

Please note: whichever sheet you designate as a “background sheet” will not appear in Play mode, printouts, or exported files. If you would like to include the background sheet in your presentation, make a copy of it before designating it as a background sheet.