If it is anything like old gscript, the first parameter is an img index, similar to the first parameter of showimg, and the array specifies coordinates of each vertex of the polygon. For the showpoly version, the array goes {x1, y1, x2, y2, x3, y3, ..., xn, yn}, each vertex is connected to the next and the last is connected to the first and the whole is filled with white. showpoly2 is the same, except you get to specify a z coordinate after each y coordinate.
If that works, you should probably tell the
wiki about it.