Instructables

Create an "Invisible" desktop shortcut in Ubuntu

Step 2: Resize Background Picture

Picture of Resize Background Picture
If your picture is bigger than your background, or if you're stretching a smaller image to fill your background, this step is for you. If your image is exactly the same size as your screen resolution or smaller, ignore this step.

With the background picture open, select image-->scale image. Scale the image to exactly the size you saw in Screen Resolution. In my case this resolution is 1024x768.