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Short description getting across the essence of the picture. It's read as a description of the image. If there's relevant text, include the text in a clear way. So for example, if it's a screenshot of the tweet, make that clear and include the username, text, and date.
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Every image should either have a descriptive alt text or be marked as alt="" if it's either decorative or has a properly attached label. Worth noting that search engines understand alt text and that's the way they traditionally searched for images before fancier ML recognition.
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