Is this a standard icon? Is it just for this one page?
If yes for both these questions, then if can't happen without a major hack of code - not a good idea. If yes and no, then find the name of the icon and replace that icon in the Moodle pix folder, but use the same name. If no for both questions, than I have to assume there is some serious coding that will need to happen to get it to work, probably in the Theme. Good luck.