This requires you to remove the padlocks on all the paintings as the description states.
This will come with progress on the main legends levels, for the origins levels you need to scratch the tickets you win after beating a level with a certain amount of lums collected, typically for the large levels where there are 10 teenies it will be 450 lums and usually 150 for the levels with 3 teenies. To unlock a new origins level you need to reveal 3 paintings when you scratch the ticket.
I am not sure if what comes up on the scratch cards is random or not but you should be aiming to get one in each level, you get 5 for completing the world.
To give you an idea when this will unlock I had all of the origins levels complete with a ticket on each one and was working on origins with 10 left to complete when it unlocked.
Long story short it will come with progress late on in the game, and aim to get a scratch card every level.