I wrote an article about Thumbnails in Windows Explorer for PSP (Photoshop Files) years ago, and it is still very popular. Now there is another and much easier solution – provided by Thumbview.
ThumbView enables image thumbnails and tooltips in MS-Windows Explorer for filetypes that are not natively supported. It started with PCX, TGA, DDS and PNG images, but now ThumbView supports an amazing 19 image types – directly inside any Explorer window!
It is working pretty well and you can download the tool and enjoy your thumbnails for free.