How to Add Alt Text on Social Media

Alt text is a short written description attached to an image or video. Screen readers read it aloud to blind and low-vision people, it shows when an image doesn't load, and it gives platforms and search engines more to go on. Most platforms now have an alt text field, but each puts it somewhere different and allows a different length, so cross-posting the same picture usually means writing it again in every app, or skipping it. This guide covers what good alt text says, where each platform takes it, and how ShortSync lets you write it once on the Caption step and send it to every platform that supports it.

1. 1

   Describe what's in the picture

   Say what someone would need to know if they couldn't see it: the subject, what's happening and the setting. "A golden retriever catching a frisbee on a beach at sunset" says more than "dog photo". Start with the most important thing, since screen reader users can stop listening at any point.

2. 2

   Include text that's in the image

   If the image has words on it, like a quote card, an infographic or a sale banner, put those words in the alt text. Otherwise the most important part of the post is missing for anyone who can't see it.

3. 3

   Keep it to a sentence or two

   One or two plain sentences is usually right. Skip "Image of" or "Photo of": screen readers already say it's an image. Leave out hashtags and marketing lines, and don't guess who a person is unless the post says so.

4. 4

   Know each platform's limit

   Pinterest allows 500 characters; Instagram, X, Threads and Tumblr 1,000; Mastodon 1,500; Bluesky 2,000; and LinkedIn 4,086. Bluesky, Mastodon, Threads and Pinterest also take alt text on videos. Write for the shortest limit you post to, or give one platform a longer version.

5. 5

   Write it once for every platform

   In ShortSync, the Alt text card on the Caption step sends one description to every selected platform that supports it: Instagram, X, LinkedIn, Pinterest, Bluesky, Mastodon, Threads and Tumblr. On a carousel you write one per image. Text that's too long for a platform is cut to fit there, and the card says where. To give one platform its own text, click its icon under Or customize alt text per platform. The AI button can also write a first draft from the image for you to check.

• Describe the image's purpose in the post, not every detail. A product shot needs the product; the background rarely matters.
• Write alt text for each image in a carousel. A screen reader user hears each one as they swipe.
• On a video, describe what happens, briefly. Captions and a transcript are still the main way to make video accessible.
• Check AI-written alt text before posting. It's a good start, but only you know what the image is meant to show.
• Using the ShortSync API? Send alt_text for one image or alt_texts for a carousel in each platform's options.

Good alt text takes a few seconds and makes your posts work for everyone, including people using screen readers. Describe the subject and any text in the image, keep it short and watch each platform's limit. Writing it once in ShortSync and sending it to every platform removes the main reason people skip it when cross-posting.