Some user's images of StackExchange are not displayed on my mac.
The user images (avatars) are stored on various services, such as Gravatar. It is possible one or more service is either unavailable or blocked on your network.

If you use a third party advert blocker within your browser, you may find some user images are blocked. Some advert blocking software considers single-source/multi-website user images a means of tracking your activity across the Internet, and thus blocks the images from loading.