Twitter In-Stream Images are images that can be attached to any Tweet and will appear on the Twitter feeds of followers. Accepting up to 4 images per Tweet, Twitter In-Stream Images no longer count against the character count of a Tweet and are widely used to accompany news stories, articles, and event announcements. Twitter In-Stream Images can be clicked and expanded to up to 1200 x 675 pixels. JPG, PNG, GIF, and animated GIF formats are accepted.
Twitter In-Stream Images have a horizontal aspect ratio of 16:9 and are sized at 600 x 335 pixels with a maximum individual image size of 5 MB.