Right now there isn't a way to do that.  Wordpress doesn't include the ability to assign thumbnails to categories.

However, we do have a new feature in our coming release that WILL do exactly as you're talking about.  We've got a new interface for letting you upload thumbnail images for categories. 

It'll probably be ready in a couple of weeks.  It's close, but we're just working on finishing up a few other features before the release.

I'll make an announcement when it's in the download center.

With the current version of Market, only the newest product thumbnails show up on the main homepage.  You can determine which items show, by editing the date of the post.  Just make the dates of the ones you want to show up, more recent than the others.

I think I understand what you're saying, but with the current version — it only has product thumbnails.  No way to do category thumbnails yet.

The code showing that is currently in the index file.

As I said above, you can designate which products show up on the homepage (regardless of category), simply by adjusting the post dates associated with them.  For example, take the 6 products you want displayed – go in and edit the posts (specifically the post dates), so that the post shows today's date.

When you save those changed posts and refresh the homepage, they should show up.