Hi, @nguyenvl159.
I guess you are forgetting something.
When we work with an INTERNAL featured image, the wordpress creates copies of that in different sizes on your server. These images are the thumbnails.
But when we work with EXTERNAL featured image, we don’t want copies of that on our server, right? So there are no thumbnails, just only external image hosted somewhere on the Internet.
For this reason I have developed the Flickr Integration feature. This way you still have external images, but there are external thumbnails to them.
Got it? =)
Ok, I understand
Thank you!!!