N[charname]M for the large portrait, N[charname]S for the small ones. You just place the new portrait in the "portraits"-directory and it will overwrite the old ones. Simply delete that portrait to reverse it to the old one again. Make sure the portraits are in the correct format (BMP) and sizes (110x170 for the large, 38x60 for the small).
Furthermore, some chars with longer names use shorter versions of their name (for example NVICONM, NVICONS for Viconia), in order to find out what these names are you can use Shadowkeeper and click "change portrait" - and there's probably a list around, too, somewheres, if you search.
EDIT Override should also work, yes. /EDIT