The Twitter / X In-stream Image size (1600×900px)
For a single image attached to a post on Twitter, now X, 1600x900 pixels at 16:9 is a reliable choice that displays fully expanded in the timeline. The platform previews inline images with a landscape crop, and a clean 16:9 file fits that preview so followers see the whole picture without tapping to expand.
X compresses uploads aggressively, so starting from a crisp 1600-pixel-wide source helps your image survive the platform's re-encoding with less visible artifacting than a smaller or oddly sized file. This width is a sweet spot: large enough to look sharp on retina displays, small enough that X does not punish it with extreme compression.
The classic trap is that the timeline preview can crop tall or square images to a wider shape, hiding the top and bottom. With a native 16:9 image that is not an issue, but if you are placing text or faces, keep them centered vertically. When you attach multiple images, X tiles them into a grid with its own crops, so single 16:9 images are the safest way to control exactly what people see.