Nonequilibrium electronic transport through a single molecule placed between a conducting substrate and a STM tip is investigated at finite voltage. The STM - molecule junction is considered as a strongly asymmetric double tunnel junction. A semi-empirical tight-binding model is used to describe linear molecules (as DNA). The coupling of the different parts of the molecule to the STM tip is a function of the distance, which results in the tunneling current dependence on the position of the STM tip (STM image of the molecule). Nonequilibrium Green Function method is applied to calculate the contribution of different molecular orbitals at finite voltage. It is shown that molecular spectra are modified by charging and vibrational effects.