Well let me sum up this application in one word - fandoodletastic. Finder does a bad bad job of caching thumbnails and when viewing a folder full of images, you will find yourself waiting a decade for Finder to generate thumbnails. But for some reason - these thumbnails are not saved anywhere - why?!
Now what CocoThumbX helps you do is to create an icon image for image, html and movie files - the image is saved into the resource fork of that file. The best thing to do is just before you go bed, drag your HD onto the App and when you wake up - your entire HD's HTML, image and movie files will be thumbnailed - just use finder to locate your file.
On a Windows machine, I had to pay for software just to view my MP4 video files as thumbnails (boo) but now I can do the same for free with CocoThumbX (yay).