You can do so through transparency effect. Select the image, click Slide Show
Custom Animation. Click Add Effect > Emphasis > Transparency. Set the start
to After Previous. Also, move the order to the top if you have other
animations added on the current slide. By doing so, your image will become
transparent during slide show.
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Shawn Toh (tohlz)
Microsoft MVP PowerPoint
Site Updated: May 19, 2007
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PowerPoint Heaven - The Power to Animate