1. An image processing apparatus comprising: an obtaining unit configured to obtain image data from a detachable memory unit; a control unit configured to execute predetermined screen processing on the image data obtained by the obtaining unit; and a presuming unit configured to estimate whether the image data stored in the detachable memory unit is image data read by a scanner based on a relationship between a vertical length and a lateral length of the image data obtained by the obtaining unit and a relationship between a vertical length and a lateral length of a regular-size sheet, wherein in a case where the presuming unit estimates that the image data stored in the detachable memory unit is image data read by the scanner, the control unit does not execute the predetermined screen processing on the image data obtained by the obtaining unit.