The Lightbox display is just too small, maximum dimension is 1054 … x ….
So a photo (clipping) larger than 1054 will not be displayed in the (true) correct size.
Sorry, that is not correct. Your “Main image size” is set to 1600px, so that is the largest you will get in the lightbox, not 1054px. Note that a lightbox will always fit an image to your screen if it is larger than the screen. I am viewing your site on a desktop that is 2160px wide, so I am seeing the full image wherever possible. E.g. your third image, “Broodexport naar Duitsland” is 1109px tall, so I see it fully in my screen.
As you have observed, it is always possible to pinch-and-zoom within the screen – it will get you to the maximum image possible. E.g. if you know that your largest image is 5000px tall, you can set the “Main image size” to 5000px. Then, an image larger than 5000px will be scaled down to 5000px, and an image smaller than 5000px will be left at its original size. E.g. your current setting is 1600px, but the “Broodexport naar Duitsland” image is 1109px tall, so you are getting an image that is 1109px in size.
Sorry, your Justified Grid, didn’t make any difference.
Maybe I am unable to understand what you are trying to do. The Justified Grid only applies to the main page – it will show you smaller thumbnails, which you can click to show the lightbox.
Is there a possibility to change (overrule) the code (CSS) for the Lightbox for Google Photo’s to:
There is no point. Photonic’s lightboxes already get you the correct size from the shortcode. As I have mentioned in my first post, if you set the “Main image size” to a larger number, that will get you the correct size. Remember that you have to pinch-and-zoom to expand the images. Maybe you don’t want to use a lightbox at all?