Let’s face it, rotating a bunch of images can be a time or a money sink. You either have to open each image one at a time, rotate them manually, and then seave them, one at a time, or you need to purchase an image editing program like Photoshop or Fireworks. And don’t get me started on figuring out how to do this in Gimp, a free, open source image editing program.
Turns out, though, you’ve already got all you need right on your Mac. Batches of image can be rotated all at once with Preview.
Find a batch of images that you need to rotate. Select them all in the Finder and open them all at once, either with a double click, or by hitting Command-O. You can also drag them all to the Preview icon in the Dock.
Once all the images are open in Preview, click on one of the image thumbnails to the left to select it. Then, go to the Edit menu and choose Select All. Or you can just hit Command-A on the keyboard.
Once they’re all selected, hit Command-R to rotate them clockwise. Hit it again to rotate them all clockwise again. Once they’re all rotated, hit Command-S to save them, or just go up to the File menu and choose Save.
Source: OS X Daily.