1. An image processing system comprising: a processor comprising hardware, wherein the processor is configured to: acquire a plurality of still images captured by an image sensor, wherein a first still image of the plurality of still images is captured as the image sensor is moved in a first moving direction, and a second still image of the plurality of still images is captured as the image sensor is moved in a second moving direction different from the first moving direction; extract a first silt image from the first still image, wherein the first slit image has a long side along a first slit direction; extract a second slit image from the second still image, wherein the second slit image has a long side along a second slit direction; extract a positioning slit from one of the plurality of still images, wherein the positioning slit has a long side along a third slit direction different from the first slit direction and the second slit direction; determine a first overlapping area between the first slit image and the positioning slit; determine a second overlapping area between the second slit image and the positioning slit; determine a third overlapping area between the first overlapping area and the second overlapping area; and generate a panoramic image based on the first slit image and the second slit image for which the third overlapping area has been determined.