Typical Microsoft - always making a simple task excessively difficult to achieve when compared to other software.
As you've discovered, Word is not a page layout/DTP program - it's a word
processor :-) But it does actually have layers. They're simply not handled
in anywhere near the same way as what you're accustomed to in InD. In fact,
you've come pretty close but haven't quite hit the target :-)
"What you're looking for is View> Header and Footer. Insert the higher res
image there using Insert> Picture> From File. Set the Text Wrapping (Format>
Picture> Layout) to Behind Text and use the features of the Advanced button to
set the position of the image Relative To: Page horizontally and vertically.
When you return to the text layer the image may appear to be dimmer than you
expect but it will print as it should."