When you add potassium bromide to potassium ferricyanide you get what is known as a rehalogenating bleach. The silver is converted to silver bromide and so the bleached image can be redeveloped if you do not fix it. This is used with some toners and intensifiers as a first step. For plain bleaching you do not need the bromide. However in this case if you over bleach you cannot bring the image back.