This paper proposes an index-based simultaneous permutation-diffusion image encryption algorithm (ISPD-IEA) based on chaos theory and a permutation-diffusion coupled encryption mechanism. The proposed method introduces a novel two-dimensional (2D) Price map derived from the Price function and classical maps, exhibiting superior chaotic dynamical properties compared to existing alternatives. By integrating the permutation-diffusion process, ISPD-IEA effectively diffuses minor changes in pixel values while altering their positions, enhancing both encryption efficiency and resistance against differential analysis attacks. Experimental results and thorough security analysis confirm the outstanding security and high encryption efficiency of ISPD-IEA. The algorithm not only achieves excellent encryption performance but also demonstrates its ability to resist various attacks commonly encountered in image encryption scenarios.