Hard photons from neutron-proton bremsstrahlung in intermediate energy heavy-ion reactions are examined as a potential probe of the nuclear symmetry energy within a transport model. Effects of the symmetry energy on the yields and spectra of hard photons are found to be generally smaller than those due to the currently existing uncertainties of both the in-medium nucleon-nucleon cross sections and the photon production probability in the elementary process $pn\to pnγ$. Very interestingly, nevertheless, the ratio of hard photon spectra $R_{1/2}(γ)$ from two reactions using isotopes of the same element is not only approximately independent of these uncertainties but also quite sensitive to the symmetry energy. For the head-on reactions of $^{132}Sn+^{124}Sn$ and $^{112}Sn+^{112}Sn$ at $E_{beam}/A=50$ MeV, for example, the $R_{1/2}(γ)$ displays a rise up to 15% when the symmetry energy is reduced by about 20% at $ρ=1.3ρ_0$ which is the maximum density reached in these reactions. ; Comment: Added new results in Fig. 6 and new references [27.28]. Phys. Lett. B in press