1. Click the slide that you want to add a background picture into.
2. On the Design tab, in the Background group (right side), click Background Styles, and then click Format Background.
3. Click on the Fill, and then select Picture or texture fill.
4. Click the File button, locate and double-click the picture which you want to insert.
Tip: Use the option “Transparency” to adjust the visibility of the picture/background, Try playing with it.