Ok, painful as this is to solve, I have a work around.
I'm new to Mac's too and although I love my machine this technical willy-waving that goes on between MS and Apple only means that users like us get caught in the middle with a lack of usable functionality. Very painful.
To get around the issue, click and drag that picture you want to your desktop (when you do this the original stays in iPhoto so don't worry; you're just creating a copy). The go into PPT and browse to the file on your desktop instead of iPhoto and you'll find you can immediately insert it into your slide deck.
Hope this helps.
Cheers
Eamus