I found it today, thought I should share:
You will need to edit the com.apple.iphoto.plist file, this is located in your (user specific) library/preferences folder.
Look for the setting: SlideShowAutoPanAndZoom which defaults to "YES". Set it to "NO".
For what it's worth, I also chose to set EnableBackgroundMusic to "NO".
This worked for me, hope it helps you as well. And as always, make backups before modifying system files, your mileage may vary, etc...