The images you are trying to replace probably account for such a minute amount of your bandwidth. Once a user browses to a page, the images get cached and don't get served up to the user again. If you are worried about bandwidth, I suggest you look at other bigger ticket items than little rank images and icons.
I could have sworn at least one of them didn't allow hotlinking. Oh well, anyway.... why don't you post what you were using as the replacement variable and tell us what the result was and then perhaps we can help you fix it.