So I have this nice new background image. As you can see, it would look kind of crowded with my desktop icons stacked up on top of the island on the right. Simple enough, I decide to mirror the image.My first shot was to import the picture into iPhoto and try to mirror it there. No luck. I don't know if I'm being blind, or if there's simply no mirroring function in iPhoto. I searched high and low, but all I could find was a reference to a "mirrored mosaic" slide show thing.
Okay, so now I try opening the photo in plain old Preview.app. Eureka! It mirrors my picture perfectly. But when I save the result as a new picture and open it in any other app, it looks just like the old one. Weird. Opening the new file in Preview.app again shows the picture mirrored, just like I wanted it. After even more research, I realize this is a known bug in Preview.app (See this entry at drunkenblog.com, scroll down to the "Oh, Preview.app" section).
Well, here's the tip I wanted to share with you. I wound up showing my new mirrored picture fullscreen in Preview.app and grabbing it using Screen Capture. Now I finally have a mirrored version of the original image. It does have some minor quality flaws, but I can live with that for now.
Apparently this is the only way to properly export an altered image from Preview.app.
[robg adds: This isn't a bug, it's a feature. Apple recently added a new Knowledge Base document on this very subject, Rotated images do not appear rotated in some applications. The explanation is, basically, that 10.4 supports EXIF-standard rotation, while previous versions of OS X did not. So EXIF-aware applications will show the rotated image, while others will not. Why is this a good thing? They explain:
This means that, in earlier versions, saving an image after rotating changed the actual pixels of the image file. However, this was often not desireable because rewriting the image file itself in the new orientation could degrade image quality (especially with lossy graphic formats such as JPEG). With EXIF-compatible applications, if you rotate an image, the file doesn't need to be rewritten.If you need the image rotated permanently in all applications, you'll need to run it through an image manipulation program that can rotate the actual pixels, such as Photoshop Elements, Photoshop, Graphic Converter, etc. You will then, of course, suffer possible image degradation...]

