1. An image processor comprising: a classifier configured to detect a predetermined target from an image, the classifier including a plurality of weak classifiers in which a next-stage weak classifier is configured to perform detection processing to a preceding-stage output domain to output an output domain; an image acquisition part configured to acquire a photographed image photographed by a camera; an assigned position specifying part configured to specify a position assigned by a user on the photographed image; a search image generator configured to extract a domain near a position assigned by the assigned position specifying part from the photographed image and generate a search image; a classifier performing part configured to cause the classifier to perform detection processing to the search image generated by the search image generator; a processing domain specifying part configured to, based on a detection result of the detection processing, specify a processing domain having the largest number of hierarchies to which the detection processing can be performed by each of the plurality of weak classifiers; and a registration image generator configured to extract the processing domain specified by the processing domain specifying part from the photographed image and generate a registration image.