This thread is old but included a banner that new input is welcome. So here goes.

I was editing a document created by converting a pdf to docx. Some of my editing included replacing images. In the process I observed that my new images would obscure the page number in the footer while the originals, of similar size and similarly positioned, did not. Try as I might, I could not get the new images to sit behind the footer. Searching led me here.

What I finally figured out is that the original images were png format and included transparent sections where the page number was located. Converting the new images to png and using IrfanView to make a section transparent solved the problem. It worked for me.

I gather that there are several other formats that support transparency (see here). But, as best I can tell, the image cannot be positioned behind the footer/header. Note that I think there are some games that can be played with watermarks. Perhaps there are others here that can flesh all that out.

An annoyance which made it harder to figure out why I was having the problem was that I'm unaware of a way to extract a picture from a document in it's native format. Back in the Office 2000 era I recall that you could edit a picture while in Word (in a manner similar to what is currently available in Acrobat). The default was Microsoft Photo Editor but, as I recall, you could specify what editor was used. Apparently not any more.