Well, I finally got it to work, but I needed Graphic Converter.
When I originally downloaded Graphic Converter, I was still using IE, which would always use StuffIt in Classic (which I never figured out how to stop). Needless to say, when I installed Graphic Converter, it went through Classic, but I was able to use it in OS X (three cheers for Carbon!).
Well, I deleted Graphic Converter, and then reloaded through OmniWeb. When I installed it this time, there was a new option in the preferences under Custom Icon. There was now an option to create 128x128 icons. When I checked this, all my NEW files previewed in crisp, clean, scalable icons in the Finder windows' Icon view.
Anyhoo...I was pretty certain that OS X was supposed to do this itself. I remember Mr Jobs noting this in an early demo, and reading about it otherwise. Of course, I had to use Graphic Converter, but on page 4: Aqua, of the "Welcome to Mac OS X" manual that came in the box, it says:
"The Finder shows document icons in several ways, including large, detailed icons that preview document contents"
...pointing to a window in Icon View with files that are obviously a Beach, a Forest, some Leaves,...etc., that are all TIF and JPG files.
In terms of speed, I usually have a couple folders with about 200 - 300 jpg's and such stored in it, which take about a second or two to load. Scrolling starts off slow, but once they've all been displayed, it scrolls normally. Speed is very much variable depending upon Icon size. Bigger = slower. I don't mind it though, because it's quicker for me to just see the file that I want than it is to try to remember its name. It will be interesting to see if the new 128x128 icons will speed up the loading since I keep the icons in my graphics files large. Previously, I'm guessing it would have to load the icon file (which was smaller) then do some math to scale it up. If it doesn't have to scale it up, maybe it will be faster...