[Theme Resonar] Blank Featured Image

Hi,

First, i use www-post-thumbnail to insert external image url as featured image and its work on Twenty Fifteen theme.

When i try change to Resonar theme, its not work very well, blank featured image. Just black.

Any suggestion about that? Image size or?

Thank you.

Demo Resonar : https://resonardemo.wordpress.com/
Plugin: https://gist.github.com/Giuseppe-Mazzapica/928bc22e5f49a654cf7c