The proposed cipher mechanism entails the encryption of a three - channel coloured input image into a single-channel encrypted image by means of QZS algorithm. Initially R, G and B components are processed parallelly, subsequently merging their outputs through basic mathematical operations before subjecting them to the QZS algorithm. Furthermore, an unequal modulus decomposition technique is applied explicitly to the R channel. MATLAB based mathematical simulations have been executed to verify and validate the proposed scheme, exhibiting its resilience against noise, differential and brute force attacks. Statistical analysis and Key sensitivity assessments further affirm the effectiveness of the scheme. Moreover, the scheme boasts an extended key space and is asymmetric in nature so has advantage and ease in key management.