This paper describes an image encryption algorithm that utilises a plaintext-related quantisation scheme. Various plaintext-related approaches from other algorithms are presented and their properties are briefly discussed. Main advantage of the proposed solution is the achievement of a similar behaviour like that of more complex approaches with a plaintextrelated technique used in a rather simple step such as quantisation. This design should result in a favourable computational complexity of the whole algorithm. The properties of the proposal are evaluated by a number of commonly used numerical parameters. Also, the statistical properties of a pseudo-random sequence that is quantised according to the plain image pixel intensities are investigated by tests from NIST 800-22 suite. Obtained results are compared to values reported in related works and they imply that the proposed solution produces encrypted images with comparable statistical properties but authors' design is faster and more efficient. 1 INTRODUCTION Development of the first encryption algorithms designed specifically for images dates to the second half of the 1990s [1, 2]. Since the early approaches exploited representation of digital images and perceived image encryption only as a permutation of pixel intensities and a combination with pseudo-random (PR) sequence, they were viewed as a quick and quite effective tool [2]. However, some design flaws were reported since then and the comparison with conventional encryption schemes such as Advanced Encryption Standard (AES) limited the usage of dedicated image encryption algorithms to certain specific applications. These include mainly applications where a change of data type is not possible such as: • encryption of images before embedding by means of image steganography [3, 4], • security improvement for medical images [5] and some biometric systems [6], • performance improvement in some simple biometric systems that do not enable hardware acceleration necessary for more complex encryption schemes [6]. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.