1. An image previewing system, comprising: a display unit comprising a screen configured to show an image; a face portion recognition unit configured to: recognize any human face contained in the image and determine face portions in the image if a human face or human faces exist in the image; a selecting unit configured to select one of the recognized face portions in the image; a comparing unit configured to compare the number of image pixels of the selected face portion with the resolution of the screen of the display unit and generate a result; and according to the result, a magnifying unit configured to magnify the selected face portion and display the magnified face portion on the screen; wherein the resolution of the screen is m×n pixels, and the number of image pixels of the selected face portion is M×N, where m and M are number of pixels on horizontal dimensional grid, and n and N are number of pixels on vertical dimensional grid; wherein if N≧n and M≧m, magnify the selected face portion to full-screen.