1. An apparatus for processing a digital image, comprising: an image sensor unit configured to sense a hand-drawn sketch and to convert the sensed hand-drawn sketch to the digital image; an image storage unit configured to store the converted digital image; an image display unit configured to visualize the stored digital image to a virtual hand-drawn sketch; and a controller configured to: sense, through the image sensor unit, a first hand-drawn sketch being drawn with a first sketching tool having a first sketching characteristics on a first sketching plane, store, through the image storage unit, the sensed first hand-drawn sketch as a first digital image with the first sketching characteristics, visualize, through the image display unit, the stored first digital image as a first virtual hand-drawn sketch on a second sketching plane, sense, through the image sensor unit, a second hand-drawn sketch being drawn with a second sketching tool having a second sketching characteristics on the second sketching plane while the first virtual hand-drawn sketch is visualized on the second sketching plane, and store, through the image storage unit, the sensed second hand-drawn sketch as a second digital image, wherein the second digital image is stored with the first sketching characteristics, when the second hand-drawn sketch is a continued sketch of the first hand-drawn sketch.