Alt text is one of the few genuinely free wins left in social media, and adoption is remarkably low.
What it does
Screen readers use it to describe your image to people who cannot see it. That is the primary reason to write it and it should be reason enough.
Separately, platforms use image descriptions as a text signal about content. Instagram generates automatic alt text using image recognition, which is generic and often wrong. Replacing it with a real description gives the system better information.
How to write it
Describe what is in the image, plainly, in one or two sentences. "A cast iron pan of golden focaccia topped with rosemary and cherry tomatoes, on a wooden counter." Not "delicious!!" and not a keyword list.
If text appears in the image, include it. If the image conveys something specific — a chart, a comparison, a before and after — say what it shows.
Where to find it
Instagram: advanced settings before posting, or edit an existing post. LinkedIn and X: an add-description button on the image. Pinterest: the description field does double duty. TikTok and YouTube use on-screen text and transcripts instead.
The honest framing
Do this because a chunk of your audience needs it. The classification benefit is real but secondary, and writing alt text as keyword filler serves neither purpose.