'Compute the image of ``self``.
That is, if ``self`` is the homomorphism `\phi: M \to N`, then compute
`im(\phi) = \{\phi(x) | x \in M \}`.  This is a submodule of `N`.
>>> from sympy import QQ
>>> from sympy.abc import x
>>> from sympy.polys.agca import homomorphism
>>> F = QQ.old_poly_ring(x).free_module(2)
>>> homomorphism(F, F, [[1, 0], [x, 0]]).image() == F.submodule([1, 0])
True'

def image(self):

    if (self._img is None):
        self._img = self._image()
    return self._img
