1. An image processing apparatus for processing an image based on an image data set including main image data that represents a main image, reduced image data that represents a reduced image of the main image, and color space identification information that expresses a color space related to the main image data, the main image data and the reduced image data and the color space identification information being associated with one another, wherein the color space identification information can indicate which of multiple color spaces including a prescribed standard color space and a particular color space is to be used, the image processing apparatus comprising: a data processor configured to select either one of the main image data and the reduced image data as processing target image data, and to execute processing including color space conversion to the processing target image data, wherein the data processor executes a prescribed basic color space conversion regardless of content of the color space identification information when the reduced image is the processing target, while the data processor executes a specified color space conversion utilizing a color space specified by the color space identification information when the main image is the processing target.