Here's an Automator service to [resize an image into several sizes](/files/workflow.zip). Unzip and copy the package "Multiple Sizes.workflow" to your **~/Library/Services** folder and you will get a new entry on the finder services context menu. Just right click any image and choose "Multiple Sizes" from the services menu.
For instance, if you select an image named "image.png" it will create the following files on the same folder:
- image_512.png - image_256.png - image_128.png - image_64.png - image_32.png
Feel free to open it up in Automator and modify to your needs :)