I have about 5700 folders, from '3d Carbon' till 'ZZipper'.I took 1 or more pics for each folder, and give them some tags. For example, all the Ferrari are Italian and GT, some of them are Concepts, Race, Coachbuilt, one-off, then there are some from the past and some modern.... To assign a tag to a pic I just use windows, and even if you do not launch Windows Picture Manager (it goes togheter with Vista), you can add a tag just selecting a picture.
I do like this:
1- I select a pic
2- in the bottom of the screen there are the info of the pic (such as date, classification, tags, dimension)
3- I can modify some of this info (I can add or delete tags, assign some stars, modify the date...)
4- When I recognize a German sport car, probably a one-off VW based, I open picture manager and made a quick search with my 'german' 'gt' 'past' 'vw based' 'coachbuilt' tags and I have a short list to look into.
I know there are several programs to make this.
I have also another method, but it took me some years to make it...It's an index, covering all the cars I have in my books and magazines. It's easy to sort them by year, nationality, engine.....but the hard part was to fill them in! This index (different from my folders and covering other marques) has got 6500 makers, but most of them are old ones.
