The demo is a little different from the "out of the box" setup.  Mostly the difference is that it's setup on an older installation of Wordpress.  It works fine on the newer versions, we just haven't reworked the demo yet.

The only changes on the demo that have been customized, is that the "categories" widget was enabled, and the storefront grid-view page was created.  Both of those are really easy to do, and there is documentation in the setup file.

As far as color customizations go, there are quite a few options you can change through the new "Market Theme" settings menu tab.  That makes it really easy.

As far as having one large picture on the home page, that's certainly not a problem.  In your "Welcome" post, you'd simply use the Wordpress toolbar to upload your image.

You'll find it very easy to use, but if you do run into problems -- you'll find a lot of members here happy to chip in some helpful advice.