on your server > root > images > ranks > if you have several diffrent styles then make folders with custom1 custom2 etc then when you go to your admincp > userranks > add ranks > User rank file path > images/ ranks/custom1 or just images/ranks/ hope it helped.
It doesn't matter in which directory you create it, as long as you enter the right path to the files on your server once you create the new ranks and set the image paths.