You can also do this:

Insert your photos normally (ie, onto the slide rather than as the slide b/g)

Then choose View, Master, Slide Master.  

Click on the slide number placeholder (to get a text insertion cursor) then
press Esc (this selects the placeholder itself, rather than the text in it).  

Choose Edit, Copy or press Ctrl+C

Choose View, Normal.

Now check each slide in the presentation; if a picture or other content hides
the slide number, choose Edit, Paste or press Ctrl+V (puts a copy of the slide
number placeholder on the slide itself ... the slide number will update to
reflect the slide's actual position in the show)

Between my writing all that out and your reading it, it took us longer to deal
with the explanation that it'll take you to fix the whole presentation once you
get rolling.  One hand on the Page Down key, the other ready to press Ctrl+V
and you're flyin'.  

-----------------------------------------
Steve Rindsberg, PPT MVP
PPT FAQ:  www.pptfaq.com
PPTools:  www.pptools.com
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