That's a good question and you would expect PubIt! to provide that kind of information so authors and publishers can properly prepare their ebooks for publication. But B&N and PubIt! have the worst documentation (IMO) of any major ebook distributor. It's pathetic and what little documenation there is contains errors (such as an incomplete character set table).
The answer to your question depends on what kind of file you plan to submit to PubIt!. If you want access to all of the features available for a Nook e-reader, then you'll need to submit your own ePub file and you will need to include an html cover page in it. In this case I don't think the marketing image will replace your ePub cover page. If you plan to submit a Word document and let PubIt! convert it to ePub, then you may not need to provide a cover page and your marketing image will be used to create the cover. But I don't know for sure about the latter. I've chosen to create my own ePub files.
Under ideal conditions, both Amazon KDP and PubIt! want something very similar. Both of their e-reader platforms are based on the open-source ePub collection of standards (that's right ePub is comprised of a number of standards which each focus on one part of a publication). If you can get your book into the ePub format and validate it to their standards (free validations tools are available), then you stand an excellent chance of publishing your ebook with very little formatting trouble.
The difference is that Amazon chose not to stay with the open-source ePub standard but they added a variety of features which are unique to the Kindle platform. For example, they accept the biggest extended character set of any e-reader platform, they offer more comprehensive support for HTML5 and CSS3, they use the media attribute to enable authors to know which level of formatting a Kindle e-reader can handle, they are more forgiving with html vs. css programming, etc. And they went their own way with regard to covers which actually are not clearly defined in the ePub standard.
The way the Kindle ebook system works, you identify the cover image file in the ePub opf file and the KindleGen converter creates a proper cover page for your book. If you include an html cover page in your ePub files for compatibility with other platforms like Nook, you can flag it in the opf file and KindleGen will know to ignore it so you don't wind up with two cover pages (a "no, no" for well-formatted ebooks).
The only down side to Amazon's approach—and it's very minor—is that you cannot link to the cover in an ebook. The Nook expects to see an html cover and you can link to it if you want.
What I've done is create an ePub workflow which can be used for both the Nook and Kindle. I use the same html, xml, opf, ncx files for both platforms. It makes it so much easier to prepare an ebook for multiple e-reader platforms. This means that I have to limit my use of extended characters to the smaller set that ePub officially supports or the Nook won't display them. And it means that I avoid html formatting attributes in favor of css (ePub wants all your text formatting to be handled with styles and the validators will complain if you don't).
I use a separate css file for each platform with style definitions that I've tweaked for each and I include appropriate code in my html pages to differentiate one platform from another. For example, if my html table of contents (TOC) file sees the Kindle css file, it knows to omit a link to the cover and visa versa.
I go ahead and include an html cover page for the Nook and include the appropriate lines in my opf so KindleGen will know to ignore it. The Nook has no problem with this and I'm able to use the same opf file for both platforms.
I hope that helps.
Kind regards, David