Gmail now displays images by default. No longer will you see emails arrive with the line at the top that says "Images are not displayed" with a link to display the included image or images. Google will now use its own proxy servers to serve images instead of sending them directly from an image's own external host server, which protects you from snooping eyes -- spammers, marketers, and other ne'er-do-wells -- who are attempting to gain information about you from an attached image and the server from which it came.
While a proxy server lets Google display images securely, you still may not want them showing up in your emails if you are on a slow connection or have a data cap. Thankfully, it's easy to return to the old way, where Google politely asks for your permission before displaying an image. On the General tab of Gmail's settings, locate the External content line and click the circle for "Ask before displaying external content."
And while we are on the topic, learn how to disable image previews on Twitter.