I'm not sure how I overlooked publishing a hint on this earlier, but a recent thread on the Macworld OS X forum served as a good reminder for me.
If you have a bunch of image files in the Finder which lack the custom preview icon (the small version of the image itself), check out Sugar Cube Software's pic2icon. pic2icon is a little freeware app which does nothing more than add a custom image preview to your image files -- but it does it very well!
pic2icon works via a drag-and-drop interface; you drag the images into the first tab of the application (which contains an image drp spot and progress bar) and it then starts creating the icons. The other tabs in the interface allow you to control things such as:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020705114914725