I just discovered that PowerPoint for Windows cannot display some images of a PowerPoint for Mac OS X file. Instead of displaying the image, a message reading "Need QuickTime and Tiff converter" is shown in its place. This came to a surprise, as I only used gif, jpeg and png files in that PowerPoint file as source images. Each of these files should be easily readable on both systems.
As it turned out, this only happened when I used the PowerPoint templates for picture slides on my Mac to make the presentation. These slide templates show a little box where pictures can be dropped in. Pictures are then automatically resized to the dimensions of this box/placeholder. During this process, they seem to be converted in to a pict file, which cannot be dislpayed on Windows machines.
So the hint is: Do not use the picture templates slides in PowerPoint for Mac. Instead, place and resize your images by hand.
[robg adds: The picture slides are available in the Add Ojbects section of the Formatting Palette; scroll down by clicking the down arrow a couple of times, and you will see them.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070611013447160