Alt Text

What is alt text?

Alt text (alternative text) is a string attached to an image's metadata that describes what the image shows. Screen readers — assistive technology used by people with visual impairments — read alt text aloud when a user encounters an image. Search engines (Google, Bing) read alt text to understand image content for image search and topical relevance. Social media platforms (Instagram, X, LinkedIn, Bluesky) use alt text as a content classification signal for their recommendation algorithms.

The convention started on the web in HTML's alt attribute and migrated to social platforms as accessibility expectations grew. Today, every major platform supports per-image alt text and most expose it as an API field for programmatic posting.

Why alt text matters

Three benefits, ranked by impact. First, accessibility — roughly 4-5% of social media users rely on screen readers either full-time or situationally (low vision, broken screens, eyes-on-other-task). Without alt text, your image is invisible to them. Second, algorithmic signal — Instagram's algorithm treats alt text as one of the strongest topical classification inputs, alongside hashtags and caption keywords. Posts with descriptive alt text get classified more accurately and surfaced to interested audiences. Third, image SEO — Google indexes alt text for image search; well-described social media images can rank in Google Images for relevant queries.

How to write good alt text

Five rules. (1) Describe what's in the image, not what it means — 'three people laughing around a whiteboard' beats 'team brainstorming session'. (2) Keep it under 125 characters — most screen readers truncate beyond that, though longer is technically allowed. (3) Don't start with 'image of' or 'picture of' — screen readers already announce that's an image. (4) Include text that's in the image — if the photo has a sign, quote, or chart label, write it out in alt text. (5) Be specific without being verbose — 'red Honda Civic in a parking lot at sunset' beats 'a car'.

Bad: 'photo of food'. Good: 'overhead shot of a pepperoni pizza on a wooden board with a glass of red wine'.

Per-platform alt text behavior

Instagram, LinkedIn, and Bluesky support per-image alt text via API and UI. X auto-generates alt text from images using its own ML and also accepts manual override; for image posts, manual is preferred for accuracy. Facebook supports alt text for images, including auto-generated descriptions. TikTok does not officially support alt text on photo posts (an accessibility gap). YouTube's video alt text equivalent is the description and closed captions, both critical. Pinterest treats the pin description as the alt text.

Set alt text per slide for an Instagram carousel

json

// POST /v1/posts
{
  "profile_name": "main",
  "platforms": ["instagram"],
  "post_type": "carousel",
  "media_urls": [
    "https://cdn.example.com/slide1.jpg",
    "https://cdn.example.com/slide2.jpg",
    "https://cdn.example.com/slide3.jpg"
  ],
  "description": "5 lessons from launching our SaaS in 2026.",
  "instagram_media_type": "CAROUSEL",

  // Single alt text applies to the cover slide
  "instagram_alt_text": "Cover slide with text 5 lessons from launching SaaS",

  // Per-slide alt text — index matches media_urls order
  "carousel_items": [
    { "alt_text": "Cover slide showing the headline 5 lessons from launching SaaS" },
    { "alt_text": "Bar chart titled MRR growth from month 1 to 6, showing steady increase" },
    { "alt_text": "Photo of three founders in front of a whiteboard with sketches" }
  ]
}

Frequently asked questions