Most black holes, especially ones at the heart of galaxies, are surrounded by huge clouds of dust and gas, and their own accretion disks. Also even for a bare black hole, it's gravity field won't be perfectly uniform to any image would be hugely distorted (like a lens made of poor quality uneven glass). On top of that as photons transit through the gravity field, even if that was perfectly uniform photons on slightly different paths would have their wavelengths stretched different amounts causing diffraction smearing the image. Finally the light path would have to be pretty close to the black hole to slingshot all the way round it, so the gravitational gradient is going to be very steep, meaning all of these effects are going to be very severe. In reality, even for a bare nonrotating black hole (much worse effects if it's rotating), which isn't really a thing as far as we know, the resulting signal is unlikely to be distinguishable from random noise.