If you need to create a lot of thumbnails on the Mac, you can spend hours trying to figure out the Automator... or you can just use a freeware program called ThumbsUp. With ThumbsUp, you can create thumbnails of images in bulk by just dragging and dropping. You get full control over the final image size and quality. If you are looking to batch resizing of images, this is probably the fastest way for the lay-person to do it.