Ionic distributions near charged interfaces control processes from colloidal aggregation to solvent flow in nanodevices. Such interfaces are often characterized by a jump in the permittivity, which gives rise to the surface polarization charge. This induced charge may significantly affect the ionic distributions so that efficient methods for modeling spatially varying dielectrics are needed. We formulate a method with O(NlogN) scaling for electrolytes between charged planar interfaces with asymmetric dielectric contrasts. Our approach, which builds on earlier work, is based on combining image charges with the particle-particle particle-mesh algorithm and representing uniform surface charges via an electric field. This enables simulations of complex dielectric interactions that outperform most alternative methods in speed and accuracy. To make the method practically useful, we provide guidelines-based upon careful tests-for choosing optimal simulation parameters. Explicit expressions for the electrostatic forces are given to facilitate the implementation of our algorithm in standard molecular dynamics packages.