Done!
The screenshot was being pulled automatically by lemmy from the image that @MentalEdge@sopuli.xyz used for the discussion post. I don't think it is particularly spoilery in the context of a post made in the community devoted to the show.
However, the update to lemmy 0.19.4 that happened on this instance just a couple days ago allows me to change the thumbnail url separately from the post url, so I changed it to the poster image from AniList since this post is in a general discussion community.
Nothing automated about the screenshots. It's all done manually by me. I was worried about the possibility of abuse from users trying to get the bot to post NSFW or spoilery images.
~~Shika!~~ Done!