By focusing a femtosecond laser beam onto the lateral face of a periodically poled KTP crystal, we generate a THz pulse through optical rectification of the laser pulse. The THz signal is recorded by means of a common THz time-domain technique, which allows us to obtain both the magnitude and sign of the THz waveform, and then the magnitude and phase of its spectral components thanks to a numerical Fourier-transform. By moving the laser beam along the crystal, we record a THz signal that renders for the alternative orientation of the crystal domains, with a lateral resolution as good as 10 mu m, whatever the THz wavelength. This demonstrates the potential of the Optical Rectification TeraHertz Imaging (ORTI) technique to produce sub-wavelength THz images.