If the images aren't saved to the phone, you can't see them on the phone - that's how the internet works.
App "developers" should make sure that, when their app creates a folder to store images, they add a file to that folder named .nomedia, so no images in it will be seen. If you can find the folder the images are in, and you can copy a file to it, you can find a .nomedia file and copy it to that folder. (Then restart the phone, to get Media Scan to run again.)
Google made the provision, the app "developers" have to understand how it works - and use it. (Real developers understand the environment they're working in, and now things like this.)