Background Transparency In Office 2004
Edit the slide master
Add your picture
Send it to the back of the slide by choosing Send to Back on the Layering menu.
Next, click Format Picture… in the formatting palette.
Slide the Transparency slider until a desired transparency effect is reached. You can click the Preview button in the bottom left-hand corner of the window to see the results of your chosen settings.
Above (before). Below (after)