Steve, do you have two 1280 × 1024 monitors arranged side-by-side? If so, then you are lucky that your particular images happen to fill your particular desktop *without needing any scaling*.
If you use GIMP or another program to scale one of those images (double the dimensions to 5120 × 2048; or halve them to 1280 × 512) and try to set the resulting file as your wallpaper, you will see the problem we are discussing.
Steve, do you have two 1280 × 1024 monitors arranged side-by-side? If so, then you are lucky that your particular images happen to fill your particular desktop *without needing any scaling*.
If you use GIMP or another program to scale one of those images (double the dimensions to 5120 × 2048; or halve them to 1280 × 512) and try to set the resulting file as your wallpaper, you will see the problem we are discussing.