1. An image processing apparatus, comprising: a memory that stores a background image and a character image that is separate from the background image and overlaps the background image; a processing target area determining unit that determines a processing target area in the background image, the processing target area being in a prescribed positional relation with the character image; a background feature parameter extracting unit that extracts from the processing target area a background feature parameter that includes a feature frequency showing a frequency characteristics of the processing target area; a character feature parameter extracting unit that extracts from the character image a character feature parameter that includes a feature frequency showing a frequency characteristics of the character image; a parameter determining unit that determines a parameter for use in image processing performed on the processing target area, according to a combination of the background feature parameter and the character feature parameter; and an image processor that performs image processing on the processing target area in accordance with the parameter determined by the parameter determining unit.