The default image path is "images/misc/vbulletin3_logo_white.gif", so a good place to put the new image would be in "images/misc". However, some people like to put folders in their images folder that relates to their style and then have the whole group of vbulletin folders listed in there for each style. If you are not doing that, then the path I first specified should be fine (and you can always change it later).