Extensive research is being done in order to use chromatophore cells as biosensors for various substances. In this paper, a link between the biological aspect of chromatophores and digital image/video processing techniques used for chromatophore characterization is established for this purpose. A model of the Gs--AC--PKA--granule motion-image feature signal transduction pathway is proposed, starting from the concentration of the input ligand and ending in the pigment area extracted from a microscope image. The model extends an existing system biology differential equation based model of the Gs--AC--PKA transduction pathway obtained from the Database of Quantitative Cellular Signaling (DQCS). Examples are presented to demonstrate the effectiveness of the proposed system model.