Chromium picolinate is actually not a fat burner. It's required (in small amounts) in the human body for proper glucose/insulin metabolism.
It has been shown in studies to marginally improve glucose usage in daibetics; and it has been shown to have virtually no effects, weight loss wise, in humans. However, it is good for weight loss in rats and, if memory serves, chickens.
Like many minerals, excessive amounts can be harmful. Regardless, this is an ineffective weight loss supplement for people.