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Doing so will open the Format tab at the top of the PowerPoint window.  If your PowerPoint file isn't yet open, first open it by double-clicking it. It doesn't matter which picture you double-click, since all of them will open the appropriate tab. This option is just below the Transitions tab. Clicking it will invoke a pop-up window. It's the first option at the top of the Compress Pictures window. Disabling this option will ensure that all pictures in your presentation are compressed. This option is near the bottom of the Compress Pictures window. Doing so will apply the compression settings to all images in your PowerPoint file, thereby reducing its overall size.

Summary:
Double-click an image. Click Compress Pictures. Uncheck the "Apply only to this picture" box. Click the E-mail (96 ppi) option. Click OK.