I've been wanting a quick and easy way to make pretty thumbnail images before posting them to my family weblog, and I think I just found it by using Pages.
Using the 'Moving Postcard' template, I dropped my image into the cutout. Then I selected the object that contains the cutout (not the image itself), and did a Copy. This way, you preserve the drop shadow and rotation. I made a 'New File from Clipboard' in Preview and saved it as a jpg. Done! Of course, you could change the rotation angle, drop shadow, border appearance, and probably many other parameters. Pretty and easy.
[robg adds: You'll want to delete the "We've Moved!" text from the template, of course...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050207144618686