Due to a bug in the #Discourse #ActivityPub plugin the image is included twice and with the wrong alt-text (the one that auto-generated on image upload).
The #ALT4you alt-text is the exact text of my toot at this location:
https://social.coop/@smallcircles/114884321738552453
cc @angusmcleod
(Plus the federation of this caused another nginx "504 Gateway Time-out" on SocialHub forum)