X image and video sizes, from X's own documentation

Both come from X's page on problems uploading a profile photo, which is also where X explains the header crop: monitors and browsers display the header at different proportions, so a header made at exactly 1500 by 500 can still lose a strip at the top and bottom.

• Keep text away from the top and bottom edges of the header. Anything inside those strips may not be seen.
• Leave the lower left of the header quiet. The round profile photo sits over that corner on the profile page, so text or a face there gets covered.
• Make the profile photo readable at a tiny size. It appears beside every post and reply, far smaller than 400 pixels. A face or a simple mark survives; a wordmark usually does not.

X's media documentation says a post can carry up to four photos, one animated GIF or one video. For GIFs it adds recommendations, because a GIF inside the 15 MB limit can still fail when the post is created: 1280 x 1080 pixels or smaller, no more than 350 frames, and no more than 300 million pixels in total, counting width times height times frames.

X does not publish a single recommended size for photos in posts, and how a photo is cropped in the timeline depends on how many images the post has and the screen it is viewed on. Tap a cropped image and X opens it in full, so the whole picture is never lost. Put the subject in the middle and check the post on a phone before you rely on an edge detail.

X's documentation states that these post video limits match the X app. For the file itself it recommends H.264 High Profile video at 30 or 60 frames per second with AAC LC audio, and these resolutions: 1280 x 720 for landscape, 720 x 1280 for portrait and 720 x 720 for square. Subscribers can upload 1080p and get 1080p playback; other accounts upload and play back at 720p. The aspect ratio must be between 1:3 and 3:1.

Posts often travel to other networks as images, and a blurry screenshot undoes the point. The post to image tool draws a post as a clean, high resolution PNG with your name and avatar, without like or view counts, ready for Instagram, LinkedIn or a slide.

• For X itself, post the text, not a picture of it. Text in an image cannot be searched, translated or read by a screen reader unless you add alt text.
• Add alt text to every image you post on X. It describes the image for people who cannot see it, and it is where the words in a text image belong.
• Export at twice the size you need. An image made at double resolution stays sharp on high density phone screens.

1. 01Header at 1500 x 500, nothing important in the top and bottom 60 pixels or the lower left corner.
2. 02Profile photo at 400 x 400, under 2 MB, still recognisable at thumbnail size.
3. 03Post photos under 5 MB each, in JPG, PNG, GIF or WEBP, four at most.
4. 04GIFs under 15 MB, ideally 1280 x 1080 or smaller with 350 frames or fewer.
5. 05Video inside your account's length and size limit, in H.264 with AAC audio, and an opening frame worth stopping for.
6. 06Alt text written for every image.

Images are only half of a profile that converts visitors into followers. The other half is 160 characters of text; how to write an X bio covers it.