"When you receive an email that contains externally linked images, Gmail usually doesn't display the images automatically. This behavior is designed to help protect your privacy; if we displayed the images automatically, it could potentially allow the sender of the email to see that the images are being fetched, and therefore know when you've read their message," explained Google.
Gmail changed this system so that you no longer have to whitelist contacts if you've sent them at least two messages. "We'll only show images in messages that are authenticated, so you won't have to worry about seeing images in messages where the sender's name or address is spoofed."
This means you'll see the message "Images are not displayed. Display images below" less often. If you don't like the new feature, you can always disable it in the Settings by checking "Ask before displaying external content".
It would be nice if Gmail added more features that let you manage the messages sent by your contacts: search options that restrict the messages to a certain group or all your contacts, filters that label the messages sent by your friends.
