Why is iodine used in photography?

Almost any heavy metal can be used as a toner in Photography, ... that being said, at one point or another iridium was used as a toner, producing a yellow image (much lighter than sepia) But since iridium is radioactive (only mildly- you would have to eat it or cover your skin with it to absorb more radiation than a day in the sun) there is no reason to use it except for the learning experience