For batch image resize on OS X Mountain Lion:
- Open Finder and select all the image files that need to be resized.
- Drag and drop all these selected files on the Preview icon in Dock. Alternatively, you can right-click and select Open With -> Preview.app
- Preview will open displaying the selected images as thumbnails in left sidebar.
- Select all thumbnails by either pressing [command + A] or by selecting Edit -> Select All from the menu.
- Select Tools -> Adjust Size from the menu. Choose your desired image size and you are done! The resized images are auto-saved and, by default, the image files are overwritten.