As far as I know, there is no easy or handy way to add a (full page) background image in LibreOffice. This is how you can do it.
- Open LibreOffice Draw.
- Navigate to Format –> Area –> Bitmaps –> Import and choose the image you wish to use as background image.
- Press okay to close the window, close LibreOffice Draw and open LibreOffice Writer.
- Navigate to Format –> Page –> Area and under Fill title choose Bitmap instead of None or whatever there reads.
- Choose in the list the bitmap you added using LibreOfficeDraw, and hit OK.
Those who wish to have a full page background might want to reduce margins to zero on the Page tab. If you know a better way to set a (full page) background, please send me a tweet about it.