Hello there,
I'm not entirely sure if this is the same on Office for Mac, as I found this solution using Windows.
I may have found a simple workaround, although the image needs to be scaled to the required size before inserting it.
Go to the view ribbon and select page layout.
3 Cells for headers will appear on the top of the document (also footers appear). Select one of these cells in which you want the image to appear.
Select the highlighted tab in the ribbon, this should be called design.
There will be an image icon, click on it. Now you can insert an image from file.
In the selected cell text will appear. Something like "&[Image]"
Once you click on any other cell the image will show.
You can use enter to lower the image slightly.
If the image is too large, rescale it and try again.
Hope this will work for Mac users as well!
Pieter F.