If you have a .docx or .pptx file with lots of big images, it may be a very large file that is slow to download, upload, or even just open.  You can compress the image data within Word or PowerPoint, for a much more efficient file.  

The process is identical in Word or PowerPoint

Video Tutorial

Step-by-Step

Click on any of the images in your file.  Word or PowerPoint will highlight the image, and also provide a new tab on the toolbar: Picture Format.  

Click the Picture Format tab, and an option within the Adjust section is Compress Pictures.  Click that, and a dialog appears with several options:

With the first option, you can choose to compress only the photo you clicked and highlighted, or every photo in the .docx or .pptx file.  Choose the latter if you need to compress every image in the file.

If you cropped or trimmed any of the images using the tools within MS Word or PowerPoint, you can delete the now-hidden or cropped portions of the images.  This is recommended, although you probably wish to ensure you have copies of the images outside the .docx or .pptx file before performing this compression.  

Lastly, choose a resolution.  As Microsoft suggests, 96 ppi will make for the smallest file, but the image quality may suffer more than you like.  330 or 220 leave images looking great but may not compress the file as much as you need.  150 ppi, or "Web," may be a good choice for a PowerPoint presentation, a print or PDF publication.  However, experiment to discover what is appropriate for your product.  You can use the Undo function (CTRL-Z or Command-Z keyboard shortcut) to reverse any compression level you choose, and try again.