how to convert tiff's to jpeg's

Quick question...I'm trying to convert tiff's into jpeg's and have tried two methods that DON'T WORK. I tried dragging them into iPhoto but a window comes up saying it doesn't recognize the file and 2) tried opening in Preview to "Save as" but the pic won't even open up in Preview.

Is there a downloadable convertor for Mac or something else I can do?

Please help!

Thanks in advance.

Braden