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Quick Featured Images Pro can not only set a default thumbnail for posts, but in general gives you more options for working with thumbnails, in particular, it gives you the ability to set the rules under which the thumbnail will be used. Quick Featured Images Pro will help you: Mass install thousands of featured images in minutes; specify automatic images by default, even with RSS importers; quick management of featured images in message lists.
Demo: https://www.quickfeaturedimages.com/