This is Minime and I am also testing how to add an alt-text to a picture.
This can be done just after uploading the picture and then choose for edit (Bewerken in Dutch).
Alt-texts for pictures are very important!! People with eye sight problems use software that reads the information for them. So be sure to add alt-texts to your pictures online.
@Marguerite have you also noticed that you can set a focal point for your image? At the same screen where you can add the description (alt) you also see dotted lines inside the image. How it works in detail can be read here: https://docs.joinmastodon.org/methods/statuses/media/#focal-points
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