Hi @VIDEOEFEKT,
I hope you are well today and thank you for your question.
You can try achieving this by adding the following code in the functions.php file of Jevelin child theme.
function custom_setup_theme() {
add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 600, 600, true );
}
add_action( 'after_setup_theme', 'custom_setup_theme', 99 );
After using the shared code pleaseĀ regenerate thumbnails using the below plugin.
https://wordpress.org/plugins/force-regenerate-thumbnails/
Best regards,
Shufflehound team