I have already managed to generate the correct thumbnails and now on the category view I have everything correctly – even without changing the images – I still have one place left and I know where the problem is – on the product page in the template only the width is set to 600px I don’t know where I can also set the height to 600px – this would solve my problem better. I just need to add a value for the height and re-generate the thumbnails.
I found something like this:
Image Sizes for Theme Developers
I think I need to use:
add_theme_support( 'woocommerce', array(
'single_image_width' => 600,
'single_image_height' => 600,
) );
I just don’t know where to put it and how ? And whether it will work.