Photoacoustic imaging theory is presented based on fundamental physical processes involved in photoacoustic phenomena. Excessive pressure is used to characterize the photoacoustic signal. A solution to photoacoustic pressure derived for liquids is investigated under a rapid heating condition. Photoacoustic image reconstruction theory is proposed for short, square heating pulses. The limitation of the reconstruction theory is discussed. Relationship of the photoacoustic image reconstruction and the Radon transform is presented. The local radiation absorption energy density is determined by the reconstruction theory for three- and two- dimensional cases and for complete or partial data acquisition. The accurate solution is an infinite series. The general formulas of the zero- and first order approximations for three- and two-dimensional cases are provided. A computer simulation for a two-dimensional case exhibits good agreement between the first order approximation and true value.