Powerpoint presentation with images can quickly become pretty heavy, making the saving process longer, occupying unnecessary space on your hard drive and eventually blocking you from sending it by email.
There is a very handy image compressing feature in Powerpoint to fix that.
Double click on any image from your PPT File to display the Image menu, and click on "Compress Pictures"
On the popup menu, uncheck the "Apply only to this picture", and select the right resolution for you. 150 ppi is usually a good balance between image quality and file size.
click OK, and it's done.
I reduced my file size by a 4.5 ratio. I can now email my PPT :)