Hi,
for this, I would use persistent data folder and load sprites as textures via the www class for example, maintain the list via the player prefs so that you don't need to scan the folder, everytime you load an image in the cache, you save that reference to your player prefs.
EasySave allows you save arrays in prefs, so you could still use an array, and you would save that array straight.
Bye,
Jean