background image not saved in document when saving in word format

First, sorry for my english, my mother tongue is french.

I do have some problems of understanding how to add a backgournd image (not linked) in a document saved in word format.

When i’m saving the document in word format (formely a JPEG inserted background) it does appear in my project first. But when i’m reopening it, it has not been saved in it.

Am i doing something wrong ?
Did i do not understood how it works ?
Is this a bug ?

My goal is to send a document including a background image as you can do it with any word document. The image is normaly saved within the document. But it does not work with LibreOffice.

I guess that if i’m linking the image to the document, it will not appear too in the saved word format, but did not tried this one as this document will be sent by email.

Some little help will be appreciated.

Regards.