I am using nextcloud, but when I see an image in the gallery and do: “copy image url”, I can see this image in another browser where I am not logged in.
So… how private is that, if anyone can see my images knowing the general nextcloud url structure?
Is there any way I can block the images from showing if the user is not logged in?
I don’t know what you exactly did but it is definitely not possible to access any file within Nextcloud without being correctly authenticated, except the file has been shared with the public.
Make sure that the file is not shared and that there is no activate session on the other PC. I just tested it on a PC by only opening a new private session and access was denied without a user authentication.
File was actually shared and another session was active on the other browser. 
Sorry.
All the best.