A new broadband monolithic THz antenna design that covers the entire 0.6 - 1.2 THz band is developed as the imaging pixel of THz focal plane array (FPA). Heterostructure backward diodes are integrated monolithically with the THz antennas and are used as the detectors to reproduce digital images from the 2-dimensional received power distribution. A new simplified design is developed which is capable of focusing the antenna beams directly onto the image plane using an extended-hemispherical lens. We show that a resolution of 1.5 mm can be achieved with this simpler design. For achieving optimal pixel sensitivity, we use full-wave antenna modeling to optimize the impedance matching between the antennas and the integrated diodes. In addition, quasi-optical propagation analysis is used to evaluate and optimize the overall imaging resolution of the THz imaging system.