1. An image processing apparatus comprising: an image processing portion configured to perform image processing on an input image in such a manner that during said image processing, size of an unprocessed image necessary for obtaining a processed image is varied depending on position of said unprocessed image in said input image; a coding portion configured to code the processed image output from said image processing portion; and a preprocessing instruction portion configured such that before image processing and coding of said input image, said preprocessing instruction portion instructs said image processing portion to perform image processing on a preprocess image which is part of said input image and sends the processed preprocess image to said coding portion to acquire necessary information for coding said input image beforehand, wherein said preprocessing instruction portion determines said preprocess image in such a manner as to reduce the size of said unprocessed image necessary for obtaining said processed preprocess image.