Automatic download of linked images must be disallowed.

When users insert images into PowerPoint presentations, they can select Link to File instead of Insert. If they do so, the image is represented by a link to a file on disk instead of being embedded in the presentation file itself. By default, when PowerPoint opens a presentation it does not display any linked images saved on a different computer unless the presentation itself is saved in a trusted location (as configured in the Trust Center). If this configuration is changed, PowerPoint will load any images that were saved in remote locations, which presents a security risk.