Computer-readable medium having an image decoding program performing: storing a reference image of a previously decoded image; receiving coded information including motion vector and rounding method information specifying a rounding method for synthesizing a prediction image of a currently decoded image; and synthesizing the prediction image via motion compensation using the motion vector information and reference image, using a positive and negative rounding method for interpolating pixel intensity values; wherein interpolation of pixel intensity values uses a rounding method specified by the rounding method information included in the encoded current P frame bitstream; wherein the rounding method information is not received from the encoded I frame bitstream; and wherein the rounding method information is included in a header section of the coded information of the currently decoded image.