I would like to slowly de-obfuscate a picture in a PPT slide show for quiz purposes. The picture will be of a person and the contenders need to guess who is slowly appearing on the screen. The effect (blur, pixellate etc.) is not extremely important. The first person correctly guessing the person in the picture wins a prize.

I have tried it with the green animation button (blur or deblur it's called I think; I don't use the English version of PPT) and set the timer to 60 seconds but even that way it is way too easy to very soon spot the person in the picture.

Is there another way I can achieve my goal?

Thanks for your help.