When you have lot of images in your PowerPoint 2011 presentation then you can reduce file size by compressing the pictures. This can be done by using the Reduce File Size option available as part of the File menu.
Step 1: Click the File menu and select Reduce File Size from the menu list.
Step 2: In the Reduce File Size screen, choose the required Picture Quality option.
Step 3: Users can also reduce the file size by removing cropped picture regions.
Step 4: If you want to apply the compress option only to selected pictures then choose the right Apply to option.
Step 5: Click OK button to confirm and apply the Reduce File Size changes.