Using high-resolution scanning tunneling microscopy and spectroscopy under magnetic field, we study the response of Dirac Landau levels to a Coulomb potential on the surface of a topological insulator Bi2Se3.Tunneling spectra show that the Landau levels shift and split in the presence of the potential variation, lifting the Landau-level degeneracy.Wave functions of Landau levels are imaged by real-space spectroscopic mapping, which delivers qualitatively different features from that of conventional electrons.By comparison with model calculations, we reveal that the two-component nature of Dirac Landau level states is manifested from the internal structure of the mapped wave functions.The modeling also suggests that the influence of Coulomb potential on the helical Dirac Landau levels brings about non-trivial spin textures in real space.