The reason why the article tells you to not use the Resources folder is exactly because it leads to the anti-pattern you want to create here: Loading multiple images from a folder in Assets before the engine decided that it acutally needs them. So if you insist on doing this, then all that advise is invalid and you can use the method described in the question.
An alternative method which might replicate your architecture in a more Unity-friendly way would be:
- Turn your PicManager into a MonoBehaviour
Sprite sprites a public variable if it isn't already
- Add a new game object with that behaviour
- Add the sprites you need to the game object through the inspector.
But there is just one scenario I could think of where this would make sense: If you have game objects which frequently change their appearance during the game. In that case you could store all the available appearances in the pic manager. When the appearance of a type of game object does not change, then it would be better to assign the sprite directly to the game object and create a prefab for each type of game object which you then instantiate whenever you need one.