Originally posted by pyn:
How can I make the icon for a .jpg file a thumbnail like .PSD files or .JPG files exported by iPhoto?
You can also do this with
Graphic Converter. Using its File > Convert menu, you can add or remove preview icons to or from, respectively, image files.
A preview can be shown as the icon of virtually any graphic file in Finder by using Icon View and selecting "Show icon preview" in View Options. However, the preview icon has to be part of the resource fork for the image to show as the icon in List or Column view.