I notice there are a lot of photos that dont fill the screen, is this because they have chosen to do this or is it because their camera cant produce images to fill the screen. I noticed my Canon G6 fits the width of my 17" monitor but it stretches virticaly slightly when i set it to desktop. Can i adjust this ?

Please forgive me i am very new to photography and i would read the manual but i had to return my camera because of a minor problem.