Solution

As was pointed out by @SatyrSack@feddit.org [1], this feature was added in this pull request. It appears that the custom thumbnail field is only relevant for posts that share a URL: The “Thumbnail URL” field is used to specify a custom thumbnail to be used in place of what would’ve been automatically generated by the provided URL [2].

Original Post

I thought it would be for adding a custom thumbnail that appears for the post in the feed, but then that seems to be a duplicate of adding an image. At any rate, I tried adding an image link to it, and no image showed up for the post. So I’m not entirely sure what it’s actually for.

EDIT (2024-11-30T22:22): Perhaps it’s for the pre-expanded thumbnail for a post if it has an image? Does that mean that it only works if you have an image specified? Can it be used to override the thumbnail that’s generated from a shared article? Does it work for a text post?

References
  1. @SatyrSack@feddit.org [To: “When creating a post, how does the “Thumbnail URL” option work exactly?”. “Kalcifer” (@Kalcifer@sh.itjust.works). sh.itjust.works. Published: 2024-11-30T22:24:10Z. Accessed: 2024-12-02T07:06Z. https://sh.itjust.works/post/28843300]. Feddit.org. Published: 2024-12-01T23:37:14Z. https://feddit.org/comment/3413077.
  2. “crates/api_crud/src/post/create.rs”. LemmyNet/Lemmy. GitHub. Published: 2024-12-01T01:14:07.000Z. Commit: 5085d1c. Accessed: 2024-12-02T06:40Z. https://github.com/LemmyNet/lemmy/blob/e49d346535f0769b85ad0fa4b0baabfbdba0deff/crates/api_crud/src/post/create.rs.
    • L91-L99
      // Only generate the thumbnail if there's no custom thumbnail provided,
// otherwise it will save it in pictrs
let generate_thumbnail = custom_thumbnail.is_none();

// Fetch post links and pictrs cached image
let metadata = fetch_link_metadata_opt(url.as_ref(), generate_thumbnail, &context).await;
let url = proxy_image_link_opt_apub(url, &context).await?;
let thumbnail_url = proxy_image_link_opt_apub(custom_thumbnail, &context)
  .await?
  .map(Into::into)
  .or(metadata.thumbnail);
      • L93: A check is done for whether a custom thumbnail URL was provided.
      • L96: If it wasn’t, then one will be generated from the shared URL.
      • L98: If a custom thumbnail URL was provided, it will be fetched from pictrs.
      • L99: If a custom thumbnail was not provided, fallback to the generated thumbnail.