Image Thumbnails not showing up on entire site. How to get it working?

Hi, I’ve finally switched my site to Hueman theme and it looks really cool.

But I can’t figure out as how to show the images in place of thumbnails, there is just an image like this (https://i.imgur.com/vEpQEZD.jpg) appearing on all pages of site.

The thumbnails only show up after editing a post and setting a featured image for it. And for me it’s not possible to edit each and every post as I’ve over 2300 posts.

Also, I think using the Regenerate thumbnails plugin won’t make them appear as there is no mention about the thumbnails size used by Hueman theme nor there is any setting in plugin options. I’m confused.

Please suggest an automated and working way that can set a featured image for all post so that no blank/grey thumbnails appear across the site.

Thanks in advance!