Disclosed are a wavelet transformation-based image encoder, decoder and method for generating a thumbnail image. In the present invention, an original image is tiled according to a preset tiling scheme, and wavelet transformation is performed for each tiled image. An LL image is extracted from the tiled image and range mapping is performed for the color values of each pixel of the extracted LL images. After the range mapping of the color values according to the pixels, a thumbnail image is generated by reconstructing the LL images according to the predetermined tiling scheme. According to the present invention as described above, when an encoder using a wavelet transformation method encodes the original image into a wavelet image or a decoder using wavelet transformation method decodes the wavelet encoded image into the original image, the thumbnail image can be easily generated.