In this paper, we introduce the quantum mechanical approach as a more physically-realistic model to accurately quantify the electron-photon interaction in Photon-induced near-field electron microscopy (PINEM). Further, we compare the maximum coupling speed between the electrons and the photons in the quantum and classical regime. For a nanosphere of radius 2.13 nm, full quantum calculations show that the maximum coupling between photon and electron occurs at a slower speed than classical calculations report. In addition, a significant reduction in PINEM field intensity is observed for the full quantum model. Furthermore, we discuss the size limitation for particles imaged using the PIMEN technique and the role of the background material in improving the PINEM intensity. We further report a significant reduction in PINEM intensity in nearly touching plasmonic particles (0.3 nm gap) due to tunneling effect.