How do I set a background image in LibreOffice Writer 5?

How do I set a page background image to a customised bitmap?

The missing step in your question is importing the bitmap using Draw

1. Import the bitmap to the "Fill" list using Draw (this step is not required in Libreoffice 5.0.1.2)
2. Set the page background image using Writer

Note:

• The lastest version of Libreoffice 5.0.1.2 has an additional "Import Graphic" button on the "Area" > "Bitmaps" dialog so step 1 above can be omitted.

Import the bitmap to the "Fill" list

• Close Writer
• Open Draw
• Menu "Format" > "Area"
• Select "Bitmaps" tab
• Click "Import"
• Choose a bitmap
• Give the bitmap a name
• Press OK

• Press OK again

  

• Here I have added a bitmap named "Desktop Background"

• Close Draw

Set the page background image

• Open Writer
• Menu "Format" > "Page"
• Select "Area" tab

• Set "Fill" to "Bitmap"

  • Press "Import Graphic" (Libreoffice 5.0.1.2)
• Select a bitmap

• Press OK

  Libreoffice 5.0.1.2:

  

  Libreoffice 5.0.0.5 (Note missing "Import Graphic" button)

  

• Here I have selected the bitmap "Desktop Background" added previously

  

The above approaches, using page format, only will fill within the page margins. For an edge-to-edge "bleeding" image, follow this approach from user cloph: https://ask.libreoffice.org/en/question/4683/full-a4-background-image-in-writer/

• insert the graphic into the document,
• choose Anchor|To Page from the context menu
• choose Wrap|In Background from the context menu

If you open Format --> Page in Libreoffice Writer 5 and go to Area tab, clicking on Import Graphic button allows you to choose your own image.

You can set size, position and tiling (repeat it multiple times in background)

Then, it will be your page background.

UPDATE: To address Jorg comment, this was made with LO v5.0.1.2.0