A "Restricted Image" is an image that will only display when a member is logged in. If you are not logged in, a placeholder image is displayed that says "Restricted". These images are flagged by the author or the system to block the display of images that might not be appropriate for public display, such as nudes or violent content.
Individual images may me marked as "restricted" by checking an option for the image when uploading or when editing the image in an Image Library. Authors can also set a profile setting to restrict the display of all of their images. Go to "My Account", "My Account Settings" for details.
The system may also mark an image as restricted if it is discovered that is contains adult or violent content. This is mostly an automated process provided by Google. They rate the probability that the image may need to be restricted.