Image Size Gridbox | WordPress.org

Hello, we use the plugin “Blog Designer Pack” and here the display form “Post Grid Box”. The image size in the plugin itself has been set to “thumbnail”. The original image size of the post images is usually 1386×924 pixels.

How can the thumbnails now be displayed rectangular (not square as currently)? Can this be solved in the plugin itself or does the original image size have to be adjusted in the blog articles themselves?

The page I need help with: [log in to see the link]