You are barking up the wrong tree. PowerPoint is a presentation application, not an image viewer. In preparation for this post, I attempted to drag & drop a selection of .jpg photograph files onto PowerPoint 2008. The application would not accept them. These photographs must be imported one-by-one.
My guess is that it is possible to write an AppleScript that responds to a selection of photographs by creating the proper template and then importing one of more photographs into each page.
Suffice it to say, there are much easier ways to create slideshows on the Mac. iPhoto will do it without requiring you to create a new file, let alone the massive file that a PowerPoint file containing 400 photographs would be. Quicktime Pro will allow you to create a slideshow from an unlimited number of photographs in one or two steps.