Go to personalize by right clicking on your desktop , then go to ' desktop background ' option .
At the bottom you will see ' picture position ' option , click it and remove it from ' tile ' to any other option .
Done ..... now you have only one picture ,
It looks like the Wallpaper is set to Tile Mode. I use Irfanview to set wallpaper. Load the image file you want. Click Options => Set As Wallpaper. Set Centered.