I had the same question. Mike answered and it was exactly what I wanted.....to choose which images I wanted on the showcase/store window.
"The only way you can control what goes in your store window is by making items featured/hidden.To do this, go to Auctiva -> Store(tab) -> Store -> Edit/Preview -> Featured Items. Items that are made hidden will not be shown in your store window and items that are featured will always be shown(as long as there aren't more than 12)".
-Mike