What type of computer, what operating system (Windows etc), what is your screen size?
What "picture" are you referring to?
Are you trying to display a photograph as your desktop background perhaps?
IF your ARE trying to display a photo as your background, on Windows 7 for example, and it does not fit the screen then:
Right click on the desktop
Choose Personalize
Choose Desktop Background
Change the "Picture position" option (bottom left) to "fill" or "fit" or "centre" or other option till the photo fits.
Of course if you had given us more info from the start we would not need to "guess"