I like seeing my previews in cover flow. I use two little programs to do this. It is very simple. Most Macs come with a neat little program called "Icon Composer". This is where mine is located, Macintosh HD>Developer>Applications>Utitlities. I use this alot, so I put it in my dock, so I can access it much quicker. The other program I use in conjunction with "Icon composer" is "Icon Dropet". Which you can download for free
here.
If you already have an image created to replace your folder icon, then you are one step ahead. Open up the "Icon composer" and drag your preferred image on to it, a little menu opens up. I always chose "copy to all smaller sizes". This will create a .ics file. I usually just save mine to the desktop. Just drag the .ics file to "Icon droplet" which will create a ".app" file and automatically saves to the location of your .ics file, which is why I save it to the desktop.
Find the folder icon you wish to change, hit CTRL I to open up the "GEt info" pane. Simply drag your new ".app" file to the icon in the upper left of the get info pane and now you will see your preview icon. Yes, it is a little extra time, but I love doing it this way because I can see all my previews in cover flow and find something I want to use much faster!!
I have a MAC running OSX 10.5.4.
I hope this helps!!
Any questions feel free to PM me,
Oh, one more thing, once you have created your new icon preview, you can trash the .ics file and the .app file, they are no longer needed, which is why I save mine to the desktop, so I can drag them right in to the trash!!