I tried (obviously these have placeholder IDs):
https://pbs.twimg.com/media/POSTID?format=jpg&name=large
https://pbs.twimg.com/media/POSTID?format=jpg
https://pbs.twimg.com/media/POSTID.jpg
and it showed the preview but said they were sample URLs
Is there any way to fix this, or is there just no way to upload X/Twitter images using links?