In PowerPoint 2003:
1>Open/Create a PowerPoint file, select one slide, Right Click, and then click Backgroud.
2>Click on Pull-Down Menu, Choose the Fill Effects... .
3>Select the Picture tab at the top of the Fill Effects dialog box. Click on the Select Picture...button to locate the photo saved on your computer.
5>Click on OK to add the picture to the slide background. You can select Apply to All or Apply button
In PowerPoint 2007:
1. Select the slide that you want to add a background picture to.
2. On the Design tab, in the Background group, click Background Styles, and then click Format Background.
3. Click Fill, and then click Picture or texture fill.
4. To insert a picture from a file, click "File", and then select the picture that you want to insert from your computer.
5. To use the picture as a background for the slides that you selected, click Close.
Here is the detail to you: http://www.ppt-to-dvd.com/learningcenter/how-to-set-up-the-different-background-for-powerpoint.html#155
Hope can help you@~