I'm an graphic artist who downloads lots and lots of preview JPEGs from commercial photograph websites. I also have a large photo collection of my own. One of my problems was that not all JPEGs have an icon that shows a picture of the file. So when I've downloaded dozens of pictures, it's sometimes hard to know which is which.
I found using Spotlight is the easy answer.
I put all my JPEGs into a folder, open the folder, and in the Spotlight search panel type jpg, and search that folder only. Presto -- up pop the JPEG icons of all my downloads as little pictures, so I can either rename or remember which is which. And of course, the same applies to other similar graphic or Photoshop files.
[robg adds: There are many third-party apps that can add image previews, saving the Spotlight step. Off the top of my head, GraphicConverter comes to mind.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=200703011424246