1. An image processing apparatus, comprising: a printing unit configured to print an image on a sheet; a reading unit configured to read the sheet; a conversion unit configured to convert a background image stored in a storage medium into a reference image that represents the background image and has a luminance value higher than that of the background image; a print control unit configured to cause the printing unit to print the reference image obtained by the conversion unit, as an entry area for a user, on the sheet; an extraction unit configured to extract an image of an area that has a luminance value lower than a first threshold or a color-difference value greater than a second threshold from an image corresponding to the entry area included in the image read by the reading unit after an entry by the user in the entry area; and a combining unit configured to combine the image extracted by the extraction unit with the background image.