If you would like to use one sheet of your diagram as the background for other diagram sheets, hover over the field that says “none” in the properties panel. It should say “Background sheet” when you hover over it. 

https://cacoo.com/diagrams/FUrIR6presRse4nL-68776.png

Select the sheet you would like to use as the background sheet. 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.

More information about using background sheets on Editor versions before 6/27/18: 

https://support.cacoo.com/hc/en-us/articles/206564318-Why-aren-t-some-sheets-being-exported-as-PDFs-even-when-I-select-All-Sheets- 

https://support.cacoo.com/hc/en-us/articles/212778448-The-Editor