With the introduction of Daily Challenges, which appear at Level 110, players can now collect posters. As you collect posters, they will be placed in your Gallery, which has it's own section in the Library of Lost Words.

You'll find the floating Gallery placard to the left of the Books: 

Before you earn a poster, you must first complete enough Daily Challenges to finish the associated puzzle. You'll see your progress in the Daily Challenges section: 

Once you've completed your poster, you'll be able to visit the Gallery at any time and see your collection of posters!  These travel posters feature Wordsworth in one of his many travels: