1. A method for generating a thumbnail image, comprising: (a) decoding an input image; (b) measuring a single Out-Of-Focus (OOF) value representing a degree of OOF of the entire decoded image; (c) determining whether or not the decoded image includes EXchangeable Image File (EXIF) information about the input image; (d) calculating, a distance based on the single measured OOF value of the entire decoded image in (b); (e) extracting a thumbnail image from the EXIF information and generating an OOF effect-applied thumbnail image by overlaying a thumbnail image shifted by the calculated distance on the extracted thumbnail image, in a presence of the EXIF information as determined in (c); and (f) generating a thumbnail image by scaling down the decoded image at a down scaler and generating an OOF effect-applied thumbnail image by overlaying a thumbnail image shifted by the calculated distance on the generated thumbnail image, in an absence of the EXIF information as determined in (c).