The Winter Graphics Set and Animals were introduced in version 1.83. The Spring Graphics Set was introduced in version 1.87. The Fall Graphics Set was introduced in version 1.89, as was an "All Seasons" bundle that will let you purchase all three seasons at a discount. The Candy Country Graphics Set was introduced in version 1.90.0.
You purchase these features from the "Decorative" tab in the in-game Store.
Once purchased, Animals and the various graphic sets apply to all your maps (not just the one you purchased it from) and to each time you play (i.e., you don't lose them when you get a "Settlement complete!"--Triple Town's version of a "Game Over"). You can also switch between the different graphics sets whenever you want.
Animals and the various graphics sets may be purchased with coins or with real money. Purchases should behave the same regardless of the method of payment used; however, please be aware that if your game data is lost (e.g., if you delete the game and later reinstall it), only purchases made with real money can be restored for free--purchases made with coins will have to be purchased again.