I discovered this feature as I discover many new things in OS X: entirely by accident. While I had a file on my desktop selected, my wrist accidentally hit the space bar on the keyboard. Instantly a quick preview window popped up giving me a preview of the image:
The feature, known as Quick Look, is one of the more than 300 new features in OS X Leopard (10.5). Quick Look also works on text files, documents, PDF files, and even HTML and PowerPoint. This method of previewing files is so much easier than opening them in their native application! (It's more than a preview though, since you can also scroll through the pages for multi-page documents.) Closing the preview window is as easy as pressing Esc, or you can simply select another file with the preview open and it will preview the newly selected file.