Yes this is a great fallback.
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Also apologies I forgot we added that other per-category social media share image setting. I kinda blacked that out due to trauma over the earlier discussion 
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When working on adding this feature I caught a bug. I noticed that right now here is how we prioritize OpenGraph image: first post image > opengraph_image > twitter_summary_large_image_url > site_apple_touch_icon_url.
And https://discourse.codinghorror.com did had the site_apple_touch_icon_url so the twitter:image tag was present <meta name="twitter:image" content="/images/default-apple-touch-icon.png" />.
Turns out Twitter does not support relative image paths. This is now fixed via:
Above commit also adds logo_url as last resort for OpenGraph/Twitter image tag. This updates opengraph image priority to: first post image > opengraph_image > twitter_summary_large_image_url > site_apple_touch_icon_url > logo_url.
Verified the fix on https://discourse.codinghorror.com:
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When set, the large_icon may be preferable than site_apple_touch_icon_ur because it’s larger. Also we did bug everyone to set it already.
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I added the large_icon as fallback. The new priority is: first post image > opengraph_image > twitter_summary_large_image_url > site_large_icon_url > site_apple_touch_icon_url > site_logo_url .
REFACTOR: Simplify finding the opengraph image · discourse/discourse@ec7f418 · GitHub
This also removes the warning that has been flooding the logs.
SiteSetting.logo_urlhas been deprecated and will be removed in the 2.4 Release. Please useSiteSetting.logoinstead
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Many of the posts in our community may contain several images.
Is there a way to tag an image or define, for a specific topic, which image to use for opengraph and social sharing?
Also, the Topic List Previews (by @merefield and @angus) plugin takes the first image in a post and uses that for the thumbnail or tile image. I’m wondering if there’s a way to tell the plugin to use the 2nd, or 3rd, or 8th image, etc.
btw @codinghorror, I got to test out that Better Insert Hyperlink feature from the 2.4 release, above, to find Topic List Previews plugin discussion, and right here with this hyperlink. Snazzy new feature!! 
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That’s already in the plugin. I added a thumbnail picker last year 
(btw, I’ve not considered how that might impact social sharing, feel free to experiment!).
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Yes – the first big image is the one that is used. So make sure the most important image is first (and large enough, tiny images won’t work).
@merefield and @codinghorror - I can confirm that using the Topic List Previews tool not only allows you to select which image you would like to use as the featured image for your thumbnails but when doing so, that image you select will also become the image that appears for opengraph and social sharing, instead of the default setting of displaying the first image in the post!
Here’s an example where I’ve chose to use the 8th image with the “Select Thumbnail” feature and here’s how the post now appears when sharing (and on my “Latest” topics page):
Here’s how to select a thumbnail image, using the Topic List Preview plugin…
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参考までに、Discourse Core は 2021年2月 からこれを実行する機能を持っています。将来的には、より発見しやすくする方法を検討するかもしれませんが、現時点では、画像のマークダウンを手動で編集し、サムネイルとして設定することができます。
デフォルトでは、Discourse はトピックの OP (最初の投稿) の最初の画像を使用します。OP の別の画像を選択したい場合は、マークダウンに |thumbnail を追加します。例：
 << これがサムネイルになります
(これは元々、公式の Topic List Thumbnails トピックで議論されていましたが、このコアのサムネイル選択機能は、そのテーマコンポーネントに限定されるものではありません)
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このトピックは2日後に自動的に閉じられました。返信はもう受け付けられません。