Dear User,
Right Click on Slide ...> Backgrounds > Click on the dropdown menu under the
picture showing box.... > Fill Effects > Click on Picture Tab .. > Click
Select Picture...
Now Browse your picture ....
Finally Apply to all for all slides
Apply for a single slide.
Go to the View menu and choose Master and Slide Master. Add your picture
there, possibly adjusting the contrast and brightness so your text shows up
on top of the picture.
--David