Did you use Insert | Page Numbers to put in the page number? In that case
it's in a frame, and I'm not quite sure what that does to the layering.
Though Page Numbers are definitely in the header/footer layer, so yes, it's
very possible nothing can go behind that.
Alternatively, keeping it in the frame might make it an object that you can
call to the front. Apparently frames can float as well, but in the text
layer.
Are you saying that Print Preview shows the page numbers on top of the
picture, but they don't print that way? That may suggest a glitch with the
printer.
What are you doing with this image? Is it a background for letterhead or
something? If so, you might want to try anchoring it in the header/footer,
in which case it would be in the same layer and might interact differently
with the page number.
The problem is that unlike in drawing programs such as Canvas or
Autocad, the graphics in Word are so poorly presented and handled it
leads to confusion.
Well yes. Because it isn't a drawing program. I don't mess with graphics
in Word, so I'm just kinda guessing above. I vaguely recall that maybe Word
has something like 19 "layers" or stories that elements can exist
within...so you see the complexity.
These might offer more information, and possibly enable a workaround.
About the draw layer
http://word.mvps.org/FAQs/DrwGrphcs/DrawLayer.htm
(hit refresh a few times in Safari, or use a different browser)
About floating graphics
http://support.microsoft.com/?kbid=268713