How can I turn a photo into a watermark?
From my understanding, the phrase “watermark” has a very restricted meaning in LibreOffice and only refers to background text inserted using Format -> Watermark. Everything else is referred to as “background image”, which is managed as per @EarnestAl’s answer.
I would recommend first of all to make a copy of your photo and then in a graphics editor reduce contrast and increase brightness so it won’t dominate the page.
In Writer click Format > Page Style. In the dialog that opens select the tab labelled Area, click Bitmap > Add/Import, navigate to and select your graphic, click Open. Make your settings for size under Styles, click Apply to see effect.
In Calc click Format > Page, same dialog will open.
In Draw click Page > Properties, same dialog will open.
Or for more control in Draw just create a new layer, add your (original) picture and adjust transparency, etc., you might have to click on image and Send to Bottom. Then Modify Layer and tick Locked
Thank you both for replies. I felt a bit foolish after discovering a method outlined on a LO help request through Google. The answer is exactly as outlined by EarnestAl and by adjusting the “transparency” you can make the photo look like a watermark.