How do I find the permanent URL (permalink) for a photo in a shared album?

I have a publicly accessible photo library and the funky URL that iCloud produces for it. However, I want to post pictures onto various photography forums that I participate on. I want to be able to get the permanent URL (often called a "permalink") for a specific picture in that shared iCloud photo album. However, if I right click and copy the URL from a photo, I can get a URL that works for a while, but then the actual URL for the photo will randomly change at some point. This means the photo I shared on the forum will now have a dead link and the photo cannot be displayed. I don't understand why iCloud randomly changes the URL, but it does. I understand this can be a problem with sites like iCloud. I also know that other sites provide a permanent URL or permalink for items like this.

Can anybody help. I've searched all over. Everybody is trying to explain the needlessly complex and obtuse photo sharing workflows for this new iCloud stuff and the Photo program on my Mac, but nobody goes beyond that to this kind of functional stuff.