This week, Google made a change to how it loads images in Gmail. Now, instead of pulling images from the source, Google caches the pictures on its own servers, and always displays images by default. That's great if you want the pictures, but if not, you can turn it off.
While Ars Technica points out that this move makes it more difficult for marketers to get data about you, pictures still take up bandwidth. If you're on a metered connection (or just don't want images enabled by default), you can disable them in your Settings here.
The privacy-minded will also note that the nature of this feature means that Google makes a copy of every image it shows you, so if you're in that rare group of people who share links to sensitive information you don't want Google storing away, you should probably turn this feature off.