Personally, I'd say attach it to the image, otherwise it makes it look like there's no alt text at all unless someone actually says "alt text in next post" or similar.

Use the alt text field. Alternative text doesn’t need to be visible for sighted users.

Edit, right alt text is an option in advanced settings under the checkboxes to share to other social media platforms, if you didn’t know.

You should write it in the image, the caption is less of a description and more of your feelings towards the picture

Probably on the image for sure. Though both wouldn’t be harmful.

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