Page content order is a concept that was introduced a while ago as a hack or bandaid, to support read-aloud for or reflow of page content even where the PDF would not be tagged properly (such that functionality based on tagged PDF could access and make availanle content from the logical structure/order).

There are lot of real world scenarios where it is impossible to sequence objects in the PDF page description such that their semantic order is reflected AND the content looks right. Like in your case, I assume the full page image only comes into play (on the semantic level) later in the page contents. As it covers the full page, once moved to its 'semantic location' in the page description, it will cover evertthng before it, because that other content – due to the nature of the PDF page dscription syntax – ends up behind it. 

My recommendation:

• forget about page content order; of someone tries to convince that you muts get page content order right, ask them how they do it when a headline straddles across two PDF pages (a typical thing in magazines with content across facing pages), or when content runs in parallel across more than one page (a typical thing for artiucles in in newspapers); don't accept if that person claims you must redesign the magazine or newspaper as hat is a cheap in unrealistic excuse. Accessibility is not about revising the real world completely, it is about providing access to the real world as it exist (we won't remove mountains just because they are difficult to climb with weel chairs, and we won't re-color tomatoes just because they are difficult to see on the tomato plant for people with red-green-blindness…)
• focus on logical order per the tagging structure in the PDF file instead
• if PAC approves of your PDF, you are already in good shape; make sure to review the HTML based report/preview in PAC, to double check whether the content order – per logical order/tagging structure – looks right (do not just trust the green check marks in PAC – these only reflect checks that can be carried out automatically by software; any accessibility checking always and by definition has to include an interactive check/rewiew by a human being).

Olaf