Twitter is testing a new way to display images in tweet timelines, which would see full images shown in tweets, as opposed to cropped previews.

The examples above show the current display format, with image cropping, on the left, and the test format on the right.
As explained by Twitter:
"Now testing on Android and iOS: when you Tweet a single image, how the image appears in the Tweet composer is how it will look on the timeline - bigger and better."
So, to clarify, this only applies to single image tweets, but the variable format could be a significant change in how your Twitter feed is displayed. The updated display would also remove the 'open for a surprise' Twitter image approach that has become something of a trend in itself, with users finding creative ways to utilize the current image cropping process.
The test will be running with a group of users on both iOS and Android, so not everyone will have the new display format.
As per Twitter's chief design officer Dantley Davis:
"People in the test will see that most Tweets with a single image in standard aspect ratio will appear uncropped when posted. People will see exactly what the image will look like in the composer tool before it's posted. Very wide or tall images will be center-cropped."
So there are still some limitations, but it could be a major change in tweet display, and a welcome one for brands and creators specifically.
In addition to this, Twitter's also testing ways for users upload and view 4K images on Android and iOS. Those in the 4K test pool, will be able to update their high-quality image preferences in their “Data usage” settings in the app.

The change could be a big update for artists looking to showcase their works via tweet.
There's some risk in this new format, in that it could make the timeline seem out of order, or it may change the uniformity of the tweet feed, which could be jarring for some users. But the full image display does look better, and could present a range of opportunities and systematic improvements, if it eventually gets a full roll-out.
We'll keep you updated on the progress of the new tests.