1. An image processing method for generating a composite image by compositing a plurality of source images, each of which has an overlapping portion with at least one another source image, comprising: a reference image acquisition step of acquiring a reference image which has overlapping portions with both of one of the source images, and another source image to be composited with that source image; a conversion generation step of generating conversions based on a hue or brightness level of the reference image for the one of the source images, and the other source image to be composited with that source image; a conversion step of converting the one of the source images, and the other source image to be composited with that source image on the basis of the generated conversions; and a composition step of generating the composite image by compositing the one of the source images, and the other source image to be composited with that source image, which have been converted.