How do I add a script to the Service menu?
While bash is cool, doesn't the image manipulation module in Automator accomplish what you are trying to achieve?

Instead of your last stage in the Automator workflow add the Photo Format module twice ( Automator Library>Photos>Change Type Of Images), the only option is target image type, where you'd presumably select jpeg for the first one and then tiff for the second one. 

This article in MacObserver explains how to use the module in general.