In easy mode you need to collect 4 items (paintings). Flowers are useless, you use them either far away to 'discard' them, or to create a path for some meeple to move to other tiles.
In hard mode you need to collect 6 items (paintings+flowers).
Note the last 4 cards of the deck make you lose if you didn't collect a specific painting. And so in hard mode, if you do the 4 paintings, you are sure to have those 4 'extra' turns to do your flowers.