From
6 Tips to Reduce the Size of your PowerPoint File
"2. How to Compress Pictures in PowerPoint: If you have any pictures in your PowerPoint presentation, then you can optimize the size by compressing pictures.
Step 1 - Click on the picture to bring up the Picture toolbar
Step 2- Click the compress picture button
Step 3 - On the Compress Pictures menu:
Apply to: All pictures in document
Change resolution: Web/Screen
Options: Compress pictures and Delete cropped areas of pictures
Click OK
When the command finishes, you can save the file at a fraction of its prior size. In this case we had 100+ maps in a 70 Meg file. After the Compress Picture command finished, the file was only 12 Meg."
Alternatively, just save the whole presentation as a series of images (.png or .jp(e)g) and use Irfanview to create an executable (.exe) slideshow.
Mac