we started the same thing some time ago.
what we do is create a MASTER CLIP / VECTOR ART FOLDER
in there we have for example major categories like , MASTER AUTOMOTIVE or MASTER SKULLS
in the MASTER SKULLS we have or create the necessary sub categories relative to skulls
example
TRIBAL (SKULLS)
SKULLS WITH FLAMES
GRUNGE SHULLS
WITH SNAKES
and so on.... we add / make as necessary any new categories when we see a skull clip we like "SKULLS WITH HEARTS".............
AUTOMOTIVE WE HAVE STUFF LIKE
CLASSIC CARS
STOCK CARS
DRAG CARS
MODERN CARS
ANTIQUE / VINTAGE CARS
CAR PARTS / ACESSORIES
from there any time we create , find, see, stumble onto , vector art clip art , google images etc that we think fits into the MASTER SKULL file we........... guess what copy it into the associated sub category of MASTER SKULLS FILE.
WE KEEP A ART DUMP FILE that we do just that dump images into on the fly to later categorize as above.
seems to work for us..it is a PITA sometimes to collect and save a file but sure pays off when we need something we have already looked for.
best of luck
mooseman