Webshots fits each image to fit your screen. However, if your monitor is not 16:9 you will get some edges of the image cropped off.
To access the settings menu open the Webshots application and click on settings tab at the top right of the page.
From there you will see an option to "Scale Webshots Desktop / Wallpaper" and Scale personal Desktop / Wallpaper. Depending on which type of image you wish to effect, change that setting to "No". Then go the Window control panel "Desktop Background" and set "Picture position" to "Fit".