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:
- Retaining original modification dates
- Adding a one-pixel border, a dog-ear corner, and/or a drop shadow
- Specifying the amount of anti-aliasing
pic2icon even shows the remaining number of images to process (via a red circle with white text, ala Mail.app) on its dock icon while it's working. Creating 115 icons took it about a minute on my G4/733, so its relatively speedy.
You can do similar things with batch conversions in GraphicConverter and PhotoShop, but I really like pic2icon's interface and ease of use. And it's tough to beat a cost of $0.00!

