Issue in your terminal the following to change the mode of how the background is displayed
gsettings set org.gnome.desktop.background picture-options "scaled"
We used «scaled» which will center the image to and will resize it to fit the screen if needed.
You could use another option if you like. To get the full list of available options issue the following in your terminal.
gsettings range org.gnome.desktop.background picture-options
It will produce a list similar to this
To reset to the default option use the following
gsettings reset org.gnome.desktop.background picture-options