Main Window - Image | Wallpaper | Fully Stretched                    Shift+Alt+F7

View Window - View | Wallpaper | Fully Stretched                    Shift+Alt+F7

Thumbnail Popup Menu - Wallpaper | Fully Stretched

 

 

This option installs the first selected file in the thumbnail view as the Windows wallpaper (like Wallpaper | Stretched) without maintaining aspect ratio.