Twitter updated its Mac client today, adding a few new features, including image previews. Image previews can potentially save you time by not forcing you to click on image links to view them in your timeline. The unfortunate side effect is that they can also clutter up your timeline.
The feature is now on the Mac, iOS, Android, and Web clients. If you prefer to view your timeline without image previews, you can disable it in the clients, except for on the Web. Jason Cipriani already showed you. Here's how to disable it on Twitter for Mac:
Go to Twitter for Mac preferences, then under General, uncheck the box next to "Image previews."