LOL, you sound like me!
I have a folder on my laptop labeled (Lin's Photo's) and then folders within folders; a folder for each plant and inside that folder another folder for each year of pictures of that plant.
I love your idea of sub-categories but I don't think the feature is available right now ... maybe some day when our wizard Dave has some free time but with six children, a working farm to run, this website, master gardener chores etc. etc. .....