Anyway, the override worked until earlier this year when Livejournal did an upgrade and then it didn't anymore. I opened a trouble ticket for the "bug" and was tld, as is often the case in software, this was "working as designed". The tech/developer suggested I choose another theme.
I re-posted asking for suggestions on a theme that could replace the header image and got back a really useful CSS override to fix my current theme. Thank you to @markf for providing the fix.
Since I had a couple of comments and an email in the past asking how I did the above. I use the Skyine theme, it is one you have to pay for. Plus I use the CSS override shown in this trouble ticket.
Note that at least as of today, this custom CSS doesn't appear to work on mobile. This may be to do with the size of the image.