aeternolab

Can you share your website URL?
One thing you need to do is target the code to each page using either the page header code injection and <style> tags, or by using each page's unique #collection-id {} with that CSS within the brackets, and repeated for each page.

Given that you're using Fluid Engine, I feel like this would actually be much easier if you just added the image to the page, and set the section to fill and align the image to the bottom, you can also adjust the image easily on mobile like that too.

This sounds like a major problem for editing, probably needs addressing by Squarespace Support before resorting to coding everything.