Twitter appears to be testing out new ways do display images to users, with some users seemingly opted into  it.  It gives priority to the actual image in the news stream, rather than the accompanying tweet like the default display shows.

Here is how photos are appearing in relation to tweets for some users:

And here is how it normally appears to most users:

From a usability perspective, with the top tweet style they are testing, the image is definitely the star of the tweet, while in the usual design placement, the tweet has equal billing with the image and people would naturally read the tweet before the image, if they are scrolling down through the feed.

Personally, I think the version they are testing looks much cleaner.  It is unknown how widespread the test is, or if Twitter plans to expand it or switch to this format completely.

It also raises the question if this change could be related to videos, as placing the videos on top of the tweet would give the video more views and engagement, especially for mobile users.  And of course, the more videos and pre-roll video advertising Twitter can show, the better for not only their bottom line but for making advertisers happy too.