I save all the high res jpg files in a folder called “Cartoons” with a folder for each year (2017, 2018 etc.) My entire hard drive is synced to Dropbox.
I also have another folder called xPSD where all the high res 600dpi PSD files are kept, and that, too, is synced to dropbox, but using Dropbox Smart Sync, it is only ever in the cloud to save hard disk space. When I need them, I download them individually.
For hard copy drawings, I have various labelled drawers. For sketches that might be something, I store them in “sketches”, for finished artwork that has been published, (and is for sale on the store) I put it in “Published”. I also have one just for my New Yorker cartoons.
How are they organised? …well. They aren’t. I just pick up the pile and shuffle through until I find the one I’m looking for. It’s not a fool-proof system, but it works for this fool.