I hope you are well today and thank you for your question.
Please regenerate the thumbnails on your site using the below plugin.
You have set too small images as featured images for the posts causing the issue as shown in the attached screenshot.
Please make sure that you are using large enough images and use the above shared plugin If you are still facing the issue.
It seems you have disabled right-click on your website so I can’t troubleshoot it.
Could you please temporarily enable the right click on your website?
Also, would you mind if I log in to your site and do some troubleshooting? If this is ok then could you please temporarily share me your site admin login details privately by adding them in the box having text “Enter your private content here (only you and forum moderators will be able to see it)”?
Do you mean it works for old posts but not new?
I have tested it on my test site and it’s working fine for me using the latest version of Gillion theme as displayed in the attached screenshot.
Please make sure that you are using below the latest version of the theme.
Please login to access this file
I logged into your website and saw you had set different sizes for the media in your website settings as shown in the attached screenshot which I changed now so it stated displaying fine.
If any image doesn’t display properly for you then please regenerate the thumbnails using the plugin suggested above.