How to Compress Your Pictures in PowerPoint 2013
Double-click any picture in the presentation to bring up the Picture Tools Format tab and then click the Compress Pictures button.
The Compress Pictures dialog box appears.
Deselect the Apply Only to This Picture option.
If you leave this option checked, only the selected picture will be compressed. Deselect this option to compress all the pictures in the presentation.
Select the Target Output option.
This step sets the compression so that image quality will still be good when the pictures are displayed on a computer screen or projector.
The images are compressed. Note that if you have a lot of pictures in your presentation, this step may take a few moments.
Save your presentation.
The images won’t actually be compressed in the presentation file until you save the file.