1. A computer-readable medium having stored thereon an image decoding program which, when executed by a computer, performs: storing a reference image which is a previously decoded image; receiving coded information including motion vector information and rounding method information specifying a rounding method used in synthesizing a prediction image of a currently decoded image; and synthesizing the prediction image by performing motion compensation using the motion vector information and the reference image; wherein the step of synthesizing a prediction image is performable using a positive rounding method and a negative rounding method for interpolating intensity values of pixels; wherein the interpolation of intensity values of pixels is performed using a rounding method specified by the rounding method information; wherein the rounding method information is included in coded information of the currently decoded image; and wherein the rounding method information specifies one of a plurality of values.