Move desktop wallpaper to the side
The Mac does not have a way (built-in) to show the desktop picture on just half of the screen.

What you could do, however is to open that picture in your picture editing program of choice, make a new picture with just a single color you like and then shrink the photo you want down a bit so it is only on the left or right side of the screen. Once it is positioned correctly save it as a JPG in your pictures folder and select that as your new desktop picture.