Here's a handy hint for Mac Lion users that will allow you see a thumbnail of recently opened documents in the Preview App. When you are in Preview, just do a three finger swipe down to be presented with a row of thumbnails across the bottom of the screen, along with any currently open documents in the screen center. From there you can easily click on the thumbnail of the image desired and it will load into Preview.
This works through App Exposé built into the Lion OS, and you may need to turn it on if not already. Just open settings, select trackpad (if you are using a MacBook) and select the "More Gestures" tab. From there be sure that the "App Exposé" three finger swipe is selected (or four finger from the dropdown, if you so wish), and you should be good to go. Now you can easily get a thumbnail of recently opened documents in Preview, which is much more convenient than trying to play the "guess the filename" game.
