Thumbnail troubleshoot

How do you add thumbnails to your post.

I have arras-theme

The website is https://www.online-movie-time.info

I tried uploading the image to the website and chmod 777 the cache folder.
Clicked on get url.

The url is https://www.online-movie-time.info/wp-content/uploads/2009/12/2012.jpg

I tried add CUSTOM FIELD. Name: thumb (default) Value: https://www.online-movie-time.info/wp-content/uploads/2009/12/2012.jpg

And it doesn’t work.

I followed this tutotial:
https://i43.tinypic.com/4ida3n.jpg