Snapshots of the flux distribution during magnetization reversal of a high-temperature superconductor Bi2Sr2CaCu 2O8+δ single crystal in the presence of artificial columnar defects were recorded using magnetic force microscopy. Thereby, the flux-antiflux boundary width and its displacement by annihilation of individual vortex-antivortex pairs could be visualized. In general the flux density follows Bean's model. However, step dislocations with a strongly anisotropic pinning behavior lead locally to a nonmonotonous flux distribution and result in a delay of the boundary propagation.