Create a letterhead from WORD

Thank you Kenavo for "leaning" (without falling) on my problem!

If I follow your advice, of course I can use my template as letterhead while keeping it for future use. The problem is that my header is still not fixed, and at the slightest false move, my photos move, the auto shapes or the pre-printed text shift, or even disappear. I would have liked the entire header to be inseparable from the template and no longer modifiable while allowing the introduction of text in Word for the body of the letter (which the "protect" function does not allow). This must be possible: all the decorative letterheads available on the internet are of this type.

Any ideas??

I don't think your problem is related to Word; I just followed your advice to create a letterhead and it prints very well. What if it's related to your printer's configuration? Sorry.

Thanks nonetheless for your help with:
In a blank document Format>Background>Patterns and Textures
Then Patterns and Textures>Image
Patterns and Textures>Image>Select an image

Simply check the box
"print background images and text"

Sorry, on Mac, I don't know. Otherwise, on PC, there is a way to avoid this printing bug:
In Photoshop or another program, create a philigram of the chosen photo (or choose a design that doesn't use too much ink).
Make sure the photo does not exceed 800X600 (not really necessary, but there's no point in heavyening the file).
In Word, place the photo in the center of the page at the desired size.
By right-clicking on the image to access the image format options, choose "text wrapping" and then "behind the text".
Hoping it won't be too different with Mac for Marinenine.