Thumbnails not showing in new theme

Hi, this is my first time here.

I used to have a website running with the Magazine Basic theme, and had it that way for a long time, so I had more or less like 300 posts. I recently had to move and get a new domain, so I installed www.ads-software.com again. I now installed the YurieWP theme in one of my attempts to renew the site. It’s all working well, except for the fact that the thumbnails are not working in the homepage (neither in the slider nor in any other part), although they show up once you click on the post. I thought it might have been due to the fact that they are not uploaded from the computer, but rather from different images’ URL, each one of them.

The thumbnails did work OK in the Magazine Basic theme, and I even installed it again in order to check if it was a problem in general or just of the theme.

I also tested by uploading an image from my PC to see if that was the problem, but it’s also not showing the thumbnail.

The site is:

Any help if very much appreciated.