How can I edit the index page created for each new site?

Focus: GridMarket MarketPress multisite.

I am building a series of pages as a “new store-template”, and hope to use new blog templates plugin to set these page up as default for each new site I create on my network.

I just realized I cannot customize the index page or even edit it. Every time someone clicks a store to go to, the index page pops up with a product grid – fine, but that will not be the “store template” homepage I prefer you see!

Maybe I am wrong, but I cannot edit that index page or wrap it into my “new store template”.

a) Is there a way to add a “store header” graphic at the top of the index page, at the very least?

b) Is there a way to replace the default index page for each new site – with my new homepage template?

Thank you.