1. An image processing method comprising: generating a tissue emphasis image by emphasizing a first tissue of at least two radiation images of different energy bands based on a difference image generated by using a variable weight which is set based on a X-ray absorption coefficient ratio of a second tissue in two different energy bands; and generating a plurality of images each corresponding to a plurality of frequency bands by performing frequency domain transform process and multi-scale decomposition process on the tissue emphasis image; generating a plurality of first images by performing noise reduction process on the plurality of images each corresponding to the plurality of frequency bands; generating a second image corresponding to the tissue emphasis image by combining the plurality of first images; and generating a diagnostic image by using at least one of the at least two radiation images of different energy bands and the second image.