OLF,
There are many extensions that will do this for you. Looking up any “new tab page” extension on the web store should give you plenty of options to work with.
The reason your background isn’t showing is because Brave’s NTP is overriding those particular (image) settings. You’ll need to get an app that overrides our NTP.
I’m going to close this thread now. For furture reference @OLF, if a thread you open with a particular issue is solved, please open new topics if you have additional issues rather than appending them to threads that are already solved.
Thank you.