Gmail started displaying images attached with mail, whenever someone you've emailed at least twice sends you a message containing images, you'll see them right away, instead previous default of "Display images below" or "Always display images from" options. If you prefer to go back to the way things were, you can choose not to display images from certain senders or from anyone. To disable images from an individual sender, click "Don't display from now on" under the "Show details" link of an email from them with images. To disable images from everybody, select "Ask before displaying external content" under "External content" on the general Settings tab.