We present a quasi-static model of the electromagnetic interaction between dipolar emitting molecule and a Plasmon-metal nano-sphere. Based on the image theory of dielectric sphere, we model the Plasmon nano-spheres by off-centered dipole images. The retardation effect is taken into account by electrodynamical modifications on spherical polarizability and dipole radiation field. The modifications of the radiative rate, total decay rate and the quantum yield for the molecules are derived. The image model indicates the strong distance dependence of the enhancement on both radiative and total decay rates. The comparison with the exact electrodynamical model and other simplified models indicate that the off-center image provide accurate predictions on radiative and total decay rates, even in close distances.