Here are your picture collections from the Explore. Once you have all items for a collection, you can turn it in to be turned into Decor for your horse's page. A random item will be chosen from the collection and you will receive that as Decor. The item chosen will be random, but rarity plays a role. For example, an Epic animal will be more difficult to obtain than a Common Animal.

Once you have received your Decor, you can equip in on your Horse's page under Manage Your Horse.