I assume you are using Windows. It does help if you tell us what version as there are differences in each Windows version.
One of the options with desktop background images is to change how Windows displays them (not sure if it works for changing images)
For Windows 7. RIGHT click on the desktop, select Personalize, then at the bottom select Dekstop Background.
In the bottom left is an option Picture Position, with options like Fill, Fit, Stretch, Tile, Centre. Try each of them and see what looks best.
This certainly works for one image, but as I say, not sure if it works for constantly changing images.