I needed a quick way to resize images in batches, and was pleased to find that Preview has that functionality. Open the whole batch of images in Preview; the images will all open in the same window with thumbnails visible in the sidebar.
Select all of the thumbnails in the sidebar (with Command-A, or click the first and then click the last while holding Shift). Next, from the Tools menu, choose Adjust Size. You'll be presented with a dialog containing options to resize to all the common 4:3 and 16:9 screen sizes, as well as the option to define custom dimensions. Images are scaled [...]