1. An X-ray imaging apparatus comprising: at least one X-ray emitter which is configured to irradiate an object with X-rays at a plurality of X-ray emission positions; an X-ray detector which is configured to detect X-rays which have been emitted by the at least one X-ray emitter and propagated through the object, and to convert the detected X-rays into an electric signal; and an image processor which is configured to acquire a plurality of original X-ray images which respectively correspond to the plurality of X-ray emission positions from the generated electric signal, and to estimate a virtual X-ray image which is acquirable at an additional X-ray emission position which is located between at least two of the plurality of X-ray emission positions, based on at least two of the plurality of original X-ray images.