In this paper, we propose a method in which PingPong256 combines LFSR and variable clock to generate an irregular PRNG and use it for image encryption. PingPong256 is guaranteed an extended period based on the two LFSRs, and the variable clock is a structure that outputs the result of operating a predetermined clock in one operation by referring to the state of the different LFSR. A variable clock is characterized by the difficulty of predicting the output at any time because the choice increases with time. PingPong256 combines the advantages of LFSR and variable clock, the convenience of hardware and software implementation, and the benefits of sensitivity and irregular periods. Also, the statistical safety was verified using the NIST SP800-22, the safety of the proposed method, and the sensitivity of the image change was tested using NPCR and UACI.