If you finish an illustration a new reward will be added to the rewards level to the map. Every illustration gives you a rewards map. But when you go to rewards map make sure you check every tile because many tiles are not playable. So check it very closely and see if you can click on it ;)
Hahaha I cleared all of it a few weeks back. Now I'll have to wait for the Lorule Map and skulltulas. Some levels were a bit tough but in general I like all of the rewards levels.