Content6 min read
How to Write Alt Text for Social Media Images
Alt text is read by people, not just crawlers. Where each platform hides the field, what a good description does, and why reused images lose theirs.
Social media alt text is a short description of a picture, read aloud by screen readers and shown when an image fails to load. Every major platform has a field for it, most of them hide it two taps deep, and the difference between a useful one and a useless one is about thirty seconds of thought.
Where each platform keeps the alt text field
| Platform | Where it is |
|---|---|
| Bluesky | An ALT badge on the image in the composer |
| Advanced settings → Write alt text, before posting | |
| X | An ALT button on the attached image |
| Threads | Tap the image in the composer |
| Add alt text on the image preview | |
| Edit photo → Alternative text |
Instagram and Facebook also generate a description automatically when you leave the field empty. It is a machine listing objects — the reason a screen reader user sometimes hears "may be an image of one person and text". Writing your own replaces it.
Bluesky is the outlier in culture rather than mechanism: the field is one tap away, the app nudges you toward it, and the norm stuck. It is the one platform where you can usually assume an image has a description worth reading.
What to write in social media alt text
Describe what the image is doing in the post, not everything in the frame.
- Lead with the point. "A line chart: signups double after the pricing change" beats
"A chart with a blue line and axis labels".
- Read the text out. If there are words in the image — a quote card, a screenshot, a
price — those words are the content. Put them in.
- Keep it to a sentence or two. A paragraph read aloud before the post itself is a
worse experience than a short description.
- Skip "image of" and "picture of". The screen reader has already said it is an
image.
- Say who, when it matters. Names, if the people are the point. Not a description of
their appearance, unless that is what the post is about.
- Decorative images need nothing. A background texture with no information has no
description to give.
What not to do with the field
Do not stuff keywords into it. Alt text carries little search weight on social platforms, and it is read aloud — a string of hashtags in the middle of a sentence is unpleasant for the person it was written for and buys nothing from anybody else.
Do not repeat the caption. If the description says exactly what the post already says, a screen reader user hears the same sentence twice and learns nothing about the picture.
Do not leave the platform's automatic description in place on anything that matters. It lists objects; it does not tell anyone why the picture is in the post.
Carry the alt text when you carry the image
This is where it usually goes wrong. Somebody finds a chart in a post, saves the picture, drops it into a deck — and the description that came with it stays behind.
The picture had a description because its author wrote one. Reusing the image and dropping it makes the copy worse for exactly the readers who needed it, at no saving whatsoever, and it takes one paste to avoid.
The Bluesky image downloader returns the alt text next to each image for this reason — Bluesky's read API hands it over, so there is no excuse for losing it.
FreeMentioned in this article
Bluesky Image DownloaderAll four images, the alt text, and the file exactly as uploaded.Other platforms are less forthcoming. What a link card publishes is the picture, not the description, so an image pulled from a Threads post or an Instagram post arrives bare. Where you can still see the original post, open it and read the description off it before you reuse the picture.
How long it is allowed to be, per platform
The field is not the same size anywhere. Instagram stores 100 characters, LinkedIn 120, Pinterest 500, X 1,000, Mastodon 1,500 and Bluesky 2,000 — a twenty-fold spread, and the overflow is cut without a warning on any of them.
That matters more than it sounds, because alt text is the one field on a post nobody proofreads. It is entered once, behind a menu, and never rendered on screen again, so a description truncated mid-sentence looks exactly like one that fit. Every limit, and what each platform does past it, is in alt text character limits.
FreeMentioned in this article
Alt Text CheckerReads your image description the way a screen reader will.The checker takes one description and a network, says whether it fits, shows where it would be cut, and runs the rules in the two sections above — the redundant opener, the caption pasted in twice, the hashtags, the untranscribed quote card.
Instagram, where the field is tightest
Instagram is worth its own treatment for two reasons: its 100-character field is the smallest of the group by a wide margin, and it is the only one of these platforms that writes a description for you if you leave the field empty.
That automatic description comes from object recognition and reads like it — "may be an image of one person, food and indoor". It is better than silence and much worse than a sentence. Yours replaces it rather than adding to it.
Where the field is on a new post, on a post that is already live, and on each slide of a carousel — plus the four faults that make an Instagram description useless — is in instagram alt text.
Why this matters beyond compliance
The accessibility case is the real one, and the WCAG text alternatives guidance sets out the standard. But there are two ordinary reasons as well.
Images fail. A slow connection, a blocked CDN, a link that has expired — when the picture does not render, the description is what is left, and a post whose entire point was in an unrendered image says nothing at all.
And writing one clarifies the post. If you cannot describe in a sentence what the image is doing there, the image may not be doing anything there.
Does alt text help SEO on social platforms?
Barely, and that is not the reason to write it. Write it for the people reading it, and take the small benefit if it exists.
How long should alt text be?
A sentence or two. Long enough to convey what the image contributes, short enough that hearing it read aloud before the post is not a chore.
Do I need alt text on every image?
Not on purely decorative ones. On anything carrying information — charts, screenshots, quote cards, anything with words in it — yes.
What happens if I leave it blank?
Instagram and Facebook generate a machine description that lists objects. It is better than silence and much worse than a sentence you wrote.
If you are reusing somebody else's image, the guide to downloading social media images covers what each platform hands over — and what it does not.
Put it to work
Tools for the job you just read about
Free to run, most of them without an account.
FreeImages & video
Bluesky Image Downloader
All four images, the alt text, and the file exactly as uploaded.
Download →4 runs
FreeContent
Alt Text Checker
Reads your image description the way a screen reader will.
Check →1 runs
FreeContent
Jargon & Acronym Detector
The words that make sense to you and not to whoever just arrived.
Detect →
FreeContent
Sentence Rhythm Analyzer
Why a draft with nothing wrong with it still reads flat.
Analyze →
FreeContent
Crutch Phrase Finder
The phrases you use every time, which you are the last to notice.
Find →1 runs
FreeContent
Weighted Character Counter
Why your editor says 240 and the post box says 287.
Count →1 runs
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