I suspect its on the unit in a hidden folder, like windows has hidden folders, and you would have to maybe turn on "show hidden files" like you do on a kindle or on a pc, or use the tool that allows you to alter the system files on the navigo instead, and that your new picture may have to be the same format and size as the existing one (like in windows)

so given that it uses win ce (5 or 6) its going to be a case of tracking it down in the system folders or root, and somehow replacing it with a new picture of the same name in the same size and format, possibly in win explorer , or maybe using wince-regedit

personally I think its too much trouble given its only a boot screen and disappears when the menus are loaded (like in windows xp) but its your choiceit never bothered me and still doesnt so I never looked into it, but it wont be on the sd card, you can be sure of that