Still on my old maps issue, when I click on the thumbnail and open the full-size image on the black background, it displays very clearly including the small print. When I click on the Title, and open the full-size image on the white background, it displays blurred.
I've cleared the caches in Zenphoto and on my browser (Edge) but it doesn't solve the problem.
Is this what is supposed to happen?

I really need to know what theme and best a link to view.

The full image actually is not compressed or resized by default unless you really tell so on the image options.
Large images - larger than the viewport - will be fake scaled to fit the display which can likely cause some blurring.

If you click on any of the images here, you will see the difference between the two full-size views. You'll also see that the plans are letterbox-shaped. The original plans would be just over 1m long.
I'm using the RCHS ROMA theme, which is ZPBootstrap very slightly modified.
I've looked at the Options >> Image pages, and the Maximum image size is set to 3000, which I think is the default. What does this number signify?

https://rchsimagearchive.org.uk/Railways-Tramways-England-Wales/Joint-Lines-Independent-and-Minor-Railways/Midland--Furness-Railway-Jt/

If you click on the thumb you get the full image with 8000x3200+ px in the Fancybox which is not processed. If you click the link you get the sized image which is of course not the full size since that would be waste on that page. So that is of course "blurred". Info about these terms: https://www.zenphoto.org/news/zenphoto-glossary/