1. An image decoding method comprising: decoding image data including information on a quantization parameter from a bitstream; determining respective quantization parameters, based on a location of each macroblock and inverse-quantizing each macroblock using the respectively determined quantization parameter, wherein macroblocks located on a boundary of a picture are inverse-quantized using a second quantization parameter obtained based on a first quantization parameter, with all other macroblocks of the picture being inverse-quantized using the first quantization parameter decoded from the bitstream; and inverse-transforming the inverse-quantized macroblocks.