1. An image decoding method comprising: decoding a picture header in a bitstream; decoding, using at least one processing device, an image in units of slices by referring to the decoded picture header, the picture header including respective information on positions and sizes of slices of at least two regions of the image that partially overlap and at least one region of the image that surrounds the at least two regions that partially overlap; and generating the image in units of the decoded slices as a part of an image by referring to the respective information on positions and sizes of the slices obtained from the decoded picture header.