To batch process your images in Preview:
- In a Finder window, select all the image files you wish to process. Double-click to open them in Preview (Note: if Preview is not set as the default application to open your image files, you can change that by selecting an image, pressing (Command + i) and changing the Open with setting to Preview. You can then select "Change all" to apply the setting to all files of that type.)
- With all the image files opened in Preview (you should see the image thumbnails appear in the right sidebar), select all the thumbnails you wish to change (Command + A to select all).
- From the Tools menu, select Adjust Size.
- Choose the settings you wish to apply to your images from the dialogue window.
- After adjusting your settings, save your images and you're all set.