1. An image-forming system that records an image through selective heat and pressure application, comprising: an image-forming sheet that includes a layer of a plurality of types of micro-capsules, each type of micro-capsule being squashed under a predetermined pressure and a predetermined temperature, said predetermined pressure and predetermined temperature of one type of micro-capsule being different from said predetermined pressure and predetermined temperature of other types of micro-capsules included in said layer, said image forming sheet being a pressure/temperature-sensitive sheet; a conveyer that moves said image-forming sheet in a transport direction; a plurality of thermal heads that correspondingly heat said types of micro-capsules to said predetermined temperature, said thermal heads extending in said transport direction and being staggeredly-aligned in said transport direction and in a line-printing direction perpendicular to said transport direction, such that an end of one of said thermal heads adjacently opposes an end of another of said thermal heads; a pressure applying unit that selectively applies pressure to said micro-capsules corresponding to said predetermined pressure, a moving apparatus that moves said thermal heads in said line-printing direction; and a driving unit that controls each of said thermal heads by a control signal corresponding to a portion of said image to be developed by said each thermal head so that said each thermal head simultaneously develops said corresponding image portion.