When you're looking at an image in Preview, you can simply hit Command-Delete to send it to the Trash.
[robg adds: As far as I can tell, this isn't documented ... and to be honest, I don't know if it's a bug or a feature. It doesn't seem to work on PDFs, but trashed PNGs and JPEGs and TIFFs that I tested it with. I also don't recall it from 10.3, so I've marked this one 10.4 only. If it works in 10.3, too, just let me know...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050513102306983