I’ve tried everything I need help today. I looked at the office help page and the instructions to me is easter egg hunting, I literally can’t find anything I have a presentation due tomorrow and this is the first time I’ve used this program.
Depending on your version it is
menu ‘View’ > item ‘Master’ > item ‘Slide Master’. Context menu (=right-click in free area) > item ‘Slide’ > item ‘Set Background Picture for Slide’
or shorter
menu ‘View’ > item ‘Master’ > item ‘Slide Master’. Context menu > item ‘Set Background Image…’