1. An image processing apparatus comprising: processing circuitry configured to specify an acquisition position of an indicator relating to blood flow on a blood vessel-containing image collected by a medical image diagnostic apparatus, and cause a display to display the acquisition position on the blood vessel-containing image and to display the indicator in association with the acquisition position, wherein the processing circuitry is configured to specify a position of a pressure sensor that is inserted into a subject from each of a plurality of X-ray images selected from a series of X-ray images for the subject, and specify a position on a reference X-ray image of the subject that corresponds to the position of the pressure sensor based on displacement of the position of the pressure sensor on the X-ray image, and cause the display to display a mark indicating an indicator derived from output of the pressure sensor or a value indicating the indicator on the reference X-rav image in a superimposed manner in accordance with a specified position on the reference X-ray image.