1. An image transmission apparatus that transmits an image to an external image display apparatus through a communication unit, the image transmission apparatus comprising: a display unit; a designation unit that designates a part of a display area of said display unit; an acquisition unit that acquires a number of horizontal pixels and a number of vertical pixels of said external image display apparatus through said communication unit; an extraction unit that extracts an image of the part of the display area designated by said designation unit if a number of horizontal pixels and a number of vertical pixels of the part of the display area designated by said designation unit are smaller than the number of horizontal pixels and the number of vertical pixels of said external image display apparatus acquired by said acquisition unit, and that extracts an image of an area smaller than the number of horizontal pixels and the number of vertical pixels of said external image display apparatus from the display area if at least one case among a case in which the number of horizontal pixels of the part of the display area designated by said designation unit is equal to or greater than the number of horizontal pixels of said external image display apparatus acquired by said acquisition unit and a case in which the number of vertical pixels of the part of the display area designated by said designation unit is equal to or greater than the number of vertical pixels of said external image display apparatus acquired by said acquisition unit is true; and a transmission unit that transmits the image extracted by said extraction unit to said external image display apparatus.