Once you have completed any of the new stages for the first time you will be given the option of replaying those stages via the 'Replay Machine' which you will find on the far right of the Garage and accessible via a small PC. Completing any of the stages again will give you a star ranking. Each stage can give between 1 to 5 stars depending on your rank (totaling to 105 stars once every stage is completed again).
Concept Art (Grocks/Bosses) 30 Stars
Concept Art (landscapes, stages, town, misc.) 50 Stars