1. An image processing method of generating an output image of a resolution higher than a resolution of an input image, using the input image, said method comprising: obtaining an edge strength of one of the input image and an image corresponding to the input image; calculating, based on the edge strength, J that is a count of data items to be used for generating a synthesized image to be used in generating the output image, where J is an integer equal to or larger than 2; generating the synthesized image by synthesizing J data items, and generating the output image using the synthesized image; enlarging the input image to generate an enlarged image; and selecting, from K first training data items in a training database, L first training data items respectively corresponding to L second training data items similar to first feature data indicating a feature of a target block included in the enlarged image, the training database storing K pairs of training data items of the K first training data items and K second training data items that are respectively associated with the K first training data items, each of the first training data items being obtained from a high-frequency component of a training image, and each of the second training data items being obtained from at least a low-frequency component of the training image, where K is an integer equal to or larger than 2, and L satisfies 2≦L≦K, wherein the data items synthesized in the synthesizing are the first training data items, and in said generating, the synthesized image is generated by synthesizing J first training data items from among the L first training data items, and the output image is generated by adding the synthesized image to the enlarged image, where J satisfies 2≦J≦L.