In this paper, we investigate the effects of asymmetric frequency-dependent joint transmit-receive I/Q imbalance on the performance of beamforming MIMO-OFDM systems. We derive the throughput-maximizing transmit/receive beamforming coefficients taking into account I/Q imbalance effects. In addition, we propose a low-complexity pilot-aided scheme for the estimation of the channel and I/Q imbalance parameters. Our simulation results demonstrate the effectiveness of the proposed generalized beamforming scheme in mitigating I/Q imbalance effects.