1. A system for causing a printing unit to print out an image including a first portion and a second portion having smaller dots than the first portion, the system comprising: a first setting unit configured to set a density of the second portion, a first causing unit configured to cause the printing unit to print out a plurality of sample images in a sheet having the first portions of a same density and the second portions of different densities, wherein only one sample image whose second portion has the density set in the first setting unit is outlined so that a user can distinctly recognize which sample image has the density set in the first setting unit; a second setting unit configured to set a density of the second portion of the image based on a user selection of the sample image; a third setting unit configured to: (a) set a content, font family information, an inclination angle and a font size of a character string, (b) set a printing order, and (c) set which of the first portion and the second portion should have a shape of the character string; a generating unit configured to generate the image including the first portion and the second portion whose densities have been set, with either the first portion or the second portion having the shape of the character string whose content, font family information, inclination angle and font size have been set; a synthesizing unit configured to synthesize the generated image and a document image so that the generated image overlaps the document image when the printing order has been set as overlap printing and configured to synthesize the generated image and a document image so that the document image overlaps the generated image when the printing order has been set as watermark printing; and a second causing unit configured to cause the printing unit to print out the synthesized image.