Where can I find puzzle pictures? When can I start assembling puzzles?

Puzzle pictures can be accessed on the side menu on the tab with the puzzle piece icon. If you do not see that icon in the game, then you have not unlocked it yet. To unlock puzzles, you should either complete any hard level (for hard levels, the level overview window has a red design and features the Clockmaker instead of townspeople) or win a puzzle piece in any event that grants it as a reward (for example, a Jewelry Box event).