Open the presentation in PowerPoint.
Click on one of the graphic images (photo perhaps) in your presentation.
A new menu option will appear, PICTURE TOOLS, FORMAT...
Click Compress Pictures
Click to deselect (disable) Apply only to this picture - this will mean all the pictures in your presentation will be compressed at the same time. If you don't do this you will have to go one by one.
For the 'Target output' select E-mail (96 ppi): minimize document size for sharing
Click File | Save As, use a different name - this way you can compare the file size and also, if need be, revert to the uncompressed presentation. When you look at the file size you should see that it's decreased.
Above is an example where I had an 11MB presentation and after compressing it came down to under 4MB. It's just an example of course, it all depends on how many images you have and how big they are.
NOTE: Compressing the images does reduce the image quality. However, I had full colour photos and I did not notice any difference in quality on the screen.
I hope that was helpful!