Either...
Go to Format>>>Background>>>The Drop Down menu of the small Background
Window>>>Fill Effects
Select the picture tab>>>select picture...choose the image>>>OK>>>OK>>>Apply
(or apply to all--- depending on what you want)
or...
Go to:
View>>>Master>>>Slide Master and then...
Format>>>Background>>>The Drop Down menu of the small Background
Window>>>Fill Effects
Select the picture tab>>>select picture...choose the image>>>OK>>>OK>>>Apply
(or apply to all--- depending on what you want)
Hope this helps!!!