We demonstrate the use of a transient polarization grating induced by two femtosecond laser pulses propagating in a LiNbO3 crystal for the generation of frequency tunable, narrow-band, picosecond THz pulses. Employing pump pulses with 150 fs duration and 2 J energy, the generated THz pulse has an energy up to 1.8 pJ, a pulse duration of 3 - 5 ps and a bandwidth of 0.1 THz. The central frequency can be tuned in the range from 0.5 to 3 THz. The generation efficiency of the transient grating method in comparison to other techniques is discussed. The possibility of shaping the temporal waveform of the THz pulses by simply shaping the spatial intensity distribution of the pump pulses is demonstrated.