1. An image input device comprising: an image capture unit which captures an image of a predetermined image capture range to produce image information; an image analysis unit which, when a range specifier for specifying a partial range within the image capture range enters the image capture range of the image capture unit, detects the range specifier based on the image information, and determines the partial range specified by the range specifier; an expansion processing unit which performs expansion process for expanding the determined partial range of the image information produced by the image capture unit when the partial range is determined by the image analysis unit; and an image information output unit which outputs the image information subjected to the expansion process to the outside, wherein the image analysis unit: detects a substantial rectangle formed by the range specifier, the substantial rectangle having sides oriented in different directions than sides of the predetermined image capture range, determines the partial range such that the partial range has a rectangular range that contains the substantial rectangle and has sides oriented in same directions as sides of the predetermined image capture range.