I was using v4.3, just upgraded to 5.0, and Help in each tells me to use Format > Page > Background tab to insert a watermark. Neither version has a Background tab in the Page Format.
How do you insert a watermark?
[Edit: added lo5 tag]
A (ridiculously) awkward workaround, at least for v4.xx (haven’t upgraded to 5 yet). Format/Page/Area allows FillType Bitmap. I’ve saved my (text) watermark as a BMP image which I can then import. The Transparency Tab will probably require something like 25%.
It took a lot of trial and error to get my BMP image just right so the watermark appears where and how I want it, but it did work.
Just want to point out that the article you link to was written in 2007, and the question specifically refers to LibreOffice 5.0. The graphics tab no longer exists. In fact, there doesn’t seem to be a way to import a bitmapped file into the page background at all.
Please take a look to this thread.
I think the second solution of adding image in the header is the more useful.
The thread you link to refers to functionality that is no longer present in LibreOffice 5.0 – it refers to a “Background” tab, which appears to have been replaced with an “Area” tab, and there is no longer a button that allows you to import graphic files into the page.
In that tab selecting bitmap, bottom-right there is an option to import a graphic.
LibreOffice 5.1 - Text Document:
Format - Page - “Area” tab
Then Fill
Choose Bitmap
At the bottom, in roughly the centre of that dialogue box, there’s an “Import Graphic” button. Select it, and import whatever you want as a background.
As Edward mentioned, you then have to go to the “Transparency” tab to choose how opaque or transparent you want it.
It doesn’t seem to be any more complicated than anything that I had to do in M$ Word. The beauty here, of course, is I get to use LibreOffice, and not M$ 
After struggling to do any of the above, I discovered that there’s a Watermark option in the printer dialogue box. For my particular printer (Epson XP300), open the dialogue box, hit properties, then page layout. There are many to choose from and you can format them in that same screen.
An [article on “LibreofficeHelp.com” site][1] tells you how you can add a transparent bookmark in LibreOffice Writer. (Tested in LibreOffice 6.x).
Short version: as of v. 6.x, you can follow Format > Watermark from the main menu to take care of this task.
[1]: https://libreofficehelp.com/how-to-add-watermark-in-libreoffice-writer/)