This study proposes an innovative method for identifying the locations of multiple simultaneous acoustic emission (AE) events in plate-like structure from the view of image processing. A synthetic aperture focusing technique (SAFT) in frequency domain for dispersive flexural wave in isotropic plate is employed to produce image displaying the locations of AE sources. Since the locations and initiation time of the AE events are unknown (namely, the length of wave signals used for SAFT is unknown), a series of images are produced by the AE wave signals with different assumed initiation times. An optimal criterion using minimum Shannon entropy is adopted to pick up the image with the locations of the AE sources and initiation time mostly approximate the actual ones. Numerical studies on an aluminum plate are performed to validate the effectiveness of the proposed optimal image-based AE source identification method.