Just a picky point about terminology...
Its not IN WGS-84, because WGS84, by itself, is not a coordinate system. It can be GCS_WGS84, ie geographic coordinates (decimal lat/long) based on WGS84.
But finding the zone is fairly easy if you know what the longitude (of maybe the centre) of the image.
If you are west (ie negative) of Greenwich, Zone # = int(abs(long) / 6) + 1; if east (like me),
Zone # = int((long + 180) / 6) + 1