Hi, there's also a third-party app letting you to align the pictures in your PowerPoint presentation, having one or several pictures per page arranged automatically and more:

http://www.norsumarketing.com.sg/ima...powerpoint.php

There is a free full featured trial version available.
For aligning the pictures specifically to top, select Edit/Settings/Top Center.

(Disclaimer: I am the author)

Cheers,
Petri