Instructables

Create an "Invisible" desktop shortcut in Ubuntu

Step 3: Select small area of background picture

Picture of Select small area of background picture
Once you've scaled the image to the right size, use the selector tool in the Gimp to select a square 48 pixels by 48 pixels. As you use the selector tool, you'll see the area in pixels displayed at the bottom of the window. You can choose any part of the picture. Ctrl+C to copy just this area to the clipboard.