Mojave shows a desert scene when I turn on my Macbook. How do I change this?

You can change the Desktop picture via the System/Desktop & Screen Saver preference pane:

This is in a file called Mojave.heic that is located in /Library/Desktop Pictures

Rename this as something else fof safekeeping and take the image you want to use and put it in this location with this exact name.

In other words, whatever image file is at

/Library/Desktop Pictures/Mojave.heic

is the one used in the login screen.

I think the OP meant the image in the login window, hence my reply. I like the one from the desert myself.

Yes it is. Of course you could always log out and see.

But that's too rational.

True. 😎

That makes sense. I don't use the login window here at home so didn't remember what it looked like. Is it the same as the daytime Mojave desert? That's my favorite also.