Boost currently does not support setting nor reading alt text on image posts.
I recently implemented an alt-text rule on one of my communities as a countermeasure to those "twitter post" style images where the content is text on an image, which is not legible by a screen reader. However, not being able to see the alt text makes it hard for me to moderate, and not being able to add it to image posts makes it hard for the users of my community to add alt text without putting it in the post body.
This missing feature could accidentally exclude visually impaired users from consuming image content produced by Boost users unless the poster thinks of adding an alt text to the post body and the visually impaired user thinks of checking the body instead of the dedicated alt text field.
We need the internet to be a more inclusive and accessible place, and this starts with helping netizens produce accessible content.