Mojave still uses Hi Sierra screen

I have an iMac 21.5" Model 'Late 2012'. I've just updated from Hi Sierra to Mojave and so far ok except that the login/Password screen does show the Mojave screen but as soon as I have signed-in the next screen that appears is the Hi Sierra screen showing the mountains not the Mojave desert. I've switched of and restarted the iMac to no avail. Any ideas? Thank you

That's the Desktop image which will remain through upgrades. So go to Sys Perf->Desktop & Screen Saver. Choose Desktop and under Apple choose Desktop Pictures. You will now have Dynamic and Desktop pictures, choose the Mojave one you want.

Raz0rEdge thank you! I've just followed your instructions and chosen a new one. Superfast reply appreciated. Have a great day.

I have a custom Desktop background and the login screen always matches the OS and then it switches to my Desktop. A few have tried to force the login background to match their regular Desktop, but it isn't supported in an easy way, so you're usually hacking some OS files to do it.

Did you get a rep point Ashwin? If so that was the perfect post, answer and grateful OP. Don't get many like that. Priceless.

He has a rep point now. Thanks for letting me know. I hadn't realised that there was that facility. You've got one too!

You get a rep point, you get a rep point, everyone gets a rep point!

Participation trophies for everyone.

You don't have your cat as the desktop image???

Raz0r Edge, I am having a similar problem to this. Except that, my desktop background won't change at all in the Admin account. I've done the "Go>Library>..." option and it didn't work. So I created a dummy account and logged into it and the background uses the Mojave backgrounds. So, I must be missing some sort of setting somewhere that is not allowing the Admin account to change the background at all. I can't even change it to a different High Sierra background! Any thoughts as to what I can try? Thanks!