It must be a signal / encryption thing? Can you confirm you can access the link every other way? Post it in a browser and it works?
Discussion
Oh wait, just the link ‘previews’? The links work?
If just previews that is how signal handles the links..
That’s what I’m thinking.
I’m just providing a 3rd party perspective and barely use signal, much less for linking image URLs.
I tested the same thing with non nostr.build image files so I’m pretty sure you’re right on it being a signal thing and not being your service (which just literally serves up the https requests and provides the images.)