From wikipedia:

[This is] a simulation of gravitational lensing caused by a Schwarzschild black hole passing in front of a background galaxy. A secondary image of the galaxy can be seen within the black hole’s Einstein radius on the side opposite the galaxy. The secondary image grows (remaining within the Einstein ring) as the primary image approaches the black hole. The surface brightness of the two images remains constant, but their angular sizes vary, hence producing an amplification of the galaxy luminosity as seen by a distant observer. Maximum amplification occurs when the galaxy (or in this case a bright part of it) is exactly behind the black hole.