Thanks for your quick response.
Your graphics card is in charge of adjusting or scaling down the pictures that you're using as your desktop wallpaper. As it turns out, if the current resolution is 1920x1080 then the bigger pictures will automatically be adjusted to fit the screens. There aren't any settings that can be set individually for each picture because it applies to the whole collection. We can suggest resizing the 3840x1080 images so it can fit the screen resolution.
Hope this helps and let us know if you have any questions.