It is not a wrapping issue, it is a layering issue. Headers and footers are in a layer behind the body of the text. By default, that is where page numbers are placed. You can insert a page number (and other text) into the body of your document (in a text box or frame if you need to move it below or above top and bottom margins). An automatic page number is a field.

You can then position your full-page image behind that text. If the full-page image is already in a header or footer, then you simply need to change the order to move it to the back.

Sections / Headers and Footers in Microsoft Word 2007-2013