Combined ultrasound and photoacoustic imaging can be implemented using a standard ultrasound imaging system interfaced with a pulsed laser source. Since in both ultrasound and photoacoustic imaging modes the acoustic waves are measured at the surface of the tissue using an ultrasound transducer, the combined imaging system can utilize the same imaging probe. However, the generation mechanisms and, therefore, the characteristics of the acoustic pressure waves in pulse-echo ultrasound and photoacoustic are different. In ultrasound imaging, the reflectivity of the tissue is the goal of the reconstruction. In photoacoustic imaging, the goal is to map the optical absorption distribution of the tissue. Photoacoustic signal is dependent on the size of the absorber while ultrasound pulseecho signal generally does not rely on the size of the reflector. In addition, the frequency response of the photoacoustic signal is usually broader compared to the ultrasound signal. Thus, wide-band transducers are required in photoacoustic imaging whereas band-limited transducer can be used in ultrasound imaging. Due to these differences in signal generation, the grayscale ultrasound beamforming or image reconstruction algorithm may not achieve the desired image quality in photoacoustic imaging. In this paper, we describe the main differences between ultrasound and photoacoustic imaging methods, and analyze the image formation algorithms in the array-based imaging system. Our numerical and experimental studies suggest that image reconstruction algorithms can be shared in combined ultrasound and photoacoustic imaging system.