How to configure fivestar's star set in Drupal 7?
Having used FiveStar with Drupal 6 and knowing where the configuration is I've had a look at FiveStar on D7 and I would suggest that because it is still Alpha the team hasn't got to moving the new features across yet as I could not find the required config either.

EDIT: I tell a lie. You can configure the star set, and as an improvement you can do it per fivestar field (i.e. you could have two ratings against a node that have different star sets). To do this go to "Manage Display" of the content type that contains your fivestar field. At the right of the field there will be a "gear" icon. Clicking on this will bring up options including the star set to use.