imwtmc uses the 2MASS Point Source Catalog, but it doesn't yet cover the entire sky. The GSC II Catalog (imwgsc2) is a visible catalog which covers the entire sky with positions which are about as good. The USNO-A2.0 catalog (imwua2) has more stars, but is based on plates taken in the 1950's, so its accuracy has been decreased by proper motion of its stars.
A set of image stars may be hand-matched to sky positions and used to get a WCS or set initial values for a reference catalog-based WCS (See example).
|Using SExtractor to find stars in an image|