What PowerPoint version are you using?
In PowerPoint 2002/XP and above, you can click Slide Show > Custom Animation.
Select the image, click Add Effect > Emphasis > Grow/Shrink.
Set the size bigger than 100% to get Grow effect, while lesser than 100% to
get Shrink effect.
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Shawn Toh (tohlz)
Microsoft MVP PowerPoint
Site Updated: Dec 24, 2006
PowerPoint Anime - Rewrite
http://pptheaven.mvps.org
PowerPoint Heaven - The Power to Animate