LessWrong should support setting empty alt text for images. There are two cases where this is the right thing to do:

• When an image is purely decorative and adds nothing to the text (like many hero images).
• When the alt text duplicates visible text (like if the image is already described by a caption).

Empty shouldn't be the default since it has a specific meaning, and this might need a warning, but right now the LessWrong editor doesn't allow it at all.