Twitter will now allow images in tweets to display in full-size rather than showing a cropped preview:
Now, when users tweet a photo taken with their iOS or Android device, it will appear in the timeline in its entirety. Plus, users can preview what an image will look like in the Tweet composer pre-post rather than having to pray to the cropping algorithm not to cut off Mr. Meow Meow's head.
As per Twitter Support:
Images with 2:1 and 3:4 aspect ratios will show in full.
It’ll be fun posting photos on Twitter. 📸