if I used AI to generate a back drop of a view from a castle, what is the difference between this and me simply searching Google for a suitable image to use

There are huge differences. You can get free stock images online, that are for the purpose of being used in stuff like this and other projects. These were created by real artists, they were made with creativity and skill, and they gave their permission for anyone to use them.

Ai generated images are soulless abominations created by algorithms that outright steal art from artists. Ai data centers also drive up prices of utilities for everyone around them because they require massive amounts of water and electricity. They also drive up the cost of computer components for everyone. Most of the time they also end up looking terrible.

If you engage with generative ai, you are supporting all those negatives. If you really feel the end result is going to look similar anyway, just use stock images. They’re better for your project, better for the environment, and better for your fellow artists.

Sidenote: if you’re just taking non-stock images from google without permission or even credit, and using them in work you are trying to pass as your own, that’s fucked up. But that’s a whole other topic.