Here's an easy way to do it without Graphic Converter, but you have to do one at a time:
1. In Preview, open a graphic file
2. Choose Copy (Command + C)
The entire graphic is now copied to the clipboard
3. In Finder, select the file's icon
4. Show Info (Command + I)
5. Select the generic graphic icon on the Show Info Panel (in 10.1, it is selected by default).
6. Choose Paste (Command + V).
The graphic image from the clipboard is now pasted in full, glorious color and detail, as the file's icon.
This is great to do just a couple files, but if you have large numbers of files that you're looking to get a preview icon for, Graphic Converter is great.