How to change a single ad image for multiple pages at once?

I have a squarespace 7.0 blog.

I want to place an ad on most blog posts. A plain image with a clickthrough, since I'm just linking to my own product.

However, I also want to change the image sometimes. Let's say I want to add/remove an "ON SALE" sticker or advertise a different product.

Changing dozens/hundreds of pages individually won't work, so is there a solution with minimal code that allows me to change the ad image for many pages or blog posts at once on Squarespace?

Back in the days of writing HTML in Notepad this would've been a trivial task (point IMG to a file and swap the file to another with the same name for any changes), but the way Squarespace handles files makes it genuinely problematic.

I would appreciate any advice, thank you.