1. I tried the obvious and it didn't work. File > Print... and hover the cursor over the Print Preview display to see the page header. Click in a header section and Menu > Insert > Choose is greyed out. No Dice!
2. Click on Done to go back to the document. Now Menu > Insert > Choose does work.
I Chose a screen shot (.png) file from my desktop. It landed on the canvas in Numbers:
With the .png file selected, Menu > Edit > Cut (command x)
Click in a header Row of a table and Menu > Edit > Paste
Now resize the cell in Format Panel > Table > Row & Column Size
With a much longer table, File > Print... and in Print Setup side panel on the right, Repeat table headers.
Image is repeated in the table header row on each page.
Not quite what you wanted, but perhaps this will work for you.
With apologies to Apple for using the Numbers icon .
I don't believe that Numbers v3 is suitable for publishing reports, invoices, etc.
Actually really not so bad for publishing, don't you think? It's a snap to publish via the internet.
Just not so great if the definition of publishing is strictly limited to the printed page-oriented publishing of the 20th century and prior.
Maybe over time we will be able to do both.
By Publishing, I mean producing something that looks professional, uniform from page to page, with precise control of scale and positioning.
But that leaves out that whole publishing on the web or to a tablet screen thing, which can cut a lot of costs out of communicating and sharing information with others, often the whole point of publishing (unless producing something like a coffee table book or museum catalog to be admired but not necessarily to be read).
Anyway, with luck, maybe in a few months time we will again have a reasonable semblance of the old print publishing plus the improved on-line stuff.