1. A non-transitory computer-readable medium storing an image processing program to make a computer execute image processing on an image group on one timeline, the image processing program making the computer execute: deriving a difference value based on a difference between pixel values of images that are adjacent in time series in the image group, the difference value indicating a degree of change between the images that are adjacent in the time series, wherein the difference value is selectable by a user from a first difference value and a second difference value, the first difference value indicative of a largest difference between the pixel values of a plurality of corresponding positions in the adjacent images and the second difference value indicative of an average value obtained by deriving the difference between the pixel values of the plurality of corresponding positions in the adjacent images and averaging the differences of all the positions; receiving a selection of one of the first difference value and the second difference value from the user; and displaying a time-series difference image showing a time-series change in the selected difference value, wherein the image processing program further makes the computer execute: displaying a timeline image corresponding to the timeline and in time series; and displaying, when an input selecting an arbitrary position on the timeline image is received, an image on the timeline corresponding to the selected position, and wherein, in the display of the time-series difference image, the time-series difference image is displayed so as to correspond to the timeline image and in time series.