1. An image synthesizing apparatus which generates, based on an original image, a synthesized image having a higher resolution than the original image, said apparatus comprising: an image acquisition unit operable to acquire the original image and plural reference images, the reference images being obtained by capturing the same object shown in the original image under differing lighting conditions; an image conversion unit operable to convert at least one of the resolution of the original image and the resolution of the plural reference images so that the resolution of the original image acquired by said image acquisition unit and the resolution of the plural reference images are the same; a conversion coefficient calculation unit operable to calculate, based on the original image and plural reference images converted to the same resolution by said image conversion unit, a conversion coefficient used in generating the original image by performing linear combination on the plural reference images; and an image synthesizing unit operable to generate plural converted reference images by converting the resolution of the plural reference images to a target resolution; generate a linear-converted image by performing linear combination on the generated plural converted reference images using the conversion coefficient calculated by said conversion coefficient calculation unit; and output, as the synthesized image, the generated linear-converted image or an adjusted linear-converted image, the adjusted linear-converted image being the linear-converted image which has been adjusted.