1. A method for compressing an image, the method comprising: generating a binary image by binarizing the image; calculating a binary pixel ratio that is a ratio of a binary pixel quantity that is a quantity of dotted pixels included in a specific area of the binary image to a total quantity of pixels included in the specific area of the binary image; generating a thin-line image by performing a line-thinning process on the specific area; calculating a thin-line pixel ratio that is a ratio of the quantity of dotted pixels included in the thin-line image to the binary pixel quantity; determining that the specific area is a map area or a photograph area based on the calculated binary pixel ratio and the calculated thin-line pixel ratio; compressing, when the specific area is determined to be a photograph area, an image of the specific area after reducing resolution of the image of the specific area; and compressing, when the specific area is determined to be a map area, the image of the specific area without reducing the resolution of the image of the specific area.