We shed new light on the long-debated question about the nature of mental images, that is, the underlying structures and processes, with a new theory of mental imagery. This theory is formalized as a computational cognitive model and provides an integrated account of the three prevalent theories of mental imagery, i.e., the descriptive, the quasi-pictorial, and the enactive theory. It does so by offering a consistent explanation for a set of empirical results, which are not plausibly provided by any of the theories individually. We give a brief review of the three theories and summarize their core commitments from a computational modeling perspective. We present a set of empirical results, the different explanations offered by the three theories, and deficiencies of their explanations. The proposed theory and model are introduced and the model's explanatory power is evaluated using the previously identified set of phenomena.