Hello,
DOCX watermarks are always added centered on the page; you can check the same with MS Word. You can scale the image added as a watermark and add watermarks per section.
If you need to add an image to even pages, the easiest approach is to create a custom DOCX template with headers and footers with an image placeholder, then import it to your DOCX and then replace the placeholder image:
https://www.phpdocx.com/api-documentation/templates/replace-placeholder-image-Word-document
Anyway, adding WordFragments with images to headers and footers work perfectly and they aren't overwrite unless you remove them.
Just to comment other approach, since phpdocx 7.5, the addImage method includes the relativeToHorizontal and relativeToVertical options to add images relative to margins, pages...:
https://www.phpdocx.com/api-documentation/word-content/insert-image-Word-document-with-PHP
Regards.