How can I convert multiple .tiff to .jpg using Preview?

Sounds like you are running os x linux, and preview doesn't support photo conversion in batch. Only Windows can do this without installing third party software. I don't know it if is still around, but search for MacBatch Tiff and download it. If you can find it, i think it was around 65 dollars to buy. If you want to do this for free, use Windows, as it has a lot more features than apple's linux, and no extra programs are needed.