https://hinchley.net/articles/using-powershell-to-automatically-change-the-desktop-wallpaper-based-on-screen-resolution/

SetWallpaper.ps1

Add-Type -TypeDefinition @"
using System;
using System.IO;
using System.Text;
using System.Drawing;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using System.Linq;
using Microsoft.Win32;

namespace WinAPI {
  class DesktopWallpaper
  {
    [StructLayout(LayoutKind.Sequential)]
    public struct Rect
    {
      public int Left;
      public int Top;
      public int Right;
      public int Bottom;
    }

    public enum DesktopSlideshowOptions
    {
      ShuffleImages = 0x01
    }

    public enum DesktopSlideshowState
    {
      Enabled = 0x01,
      Slideshow = 0x02,
      DisabledByRemoteSession = 0x04
    }

    public enum DesktopSlideshowDirection
    {
      Forward = 0,
      Backward = 1
    }

    public enum DesktopWallpaperPosition
    {
      Center = 0,
      Tile = 1,
      Stretch = 2,
      Fit = 3,
      Fill = 4,
      Span = 5
    }

    [ComImport, Guid("B92B56A9-8B55-4E14-9A89-0199BBB6F93B"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IDesktopWallpaper
    {
      void SetWallpaper([MarshalAs(UnmanagedType.LPWStr)] string monitorID, [MarshalAs(UnmanagedType.LPWStr)] string wallpaper);

      [return: MarshalAs(UnmanagedType.LPWStr)]
      string GetWallpaper([MarshalAs(UnmanagedType.LPWStr)] string monitorID);

      [return: MarshalAs(UnmanagedType.LPWStr)]
      string GetMonitorDevicePathAt(uint monitorIndex);

      [return: MarshalAs(UnmanagedType.U4)]
      uint GetMonitorDevicePathCount();

      [return: MarshalAs(UnmanagedType.Struct)]
      Rect GetMonitorRECT([MarshalAs(UnmanagedType.LPWStr)] string monitorID);

      void SetBackgroundColor([MarshalAs(UnmanagedType.U4)] uint color);

      [return: MarshalAs(UnmanagedType.U4)]
      uint GetBackgroundColor();

      void SetPosition([MarshalAs(UnmanagedType.I4)] DesktopWallpaperPosition position);

      [return: MarshalAs(UnmanagedType.I4)]
      DesktopWallpaperPosition GetPosition();

      void SetSlideshow(IntPtr items);

      IntPtr GetSlideshow();

      void SetSlideshowOptions(DesktopSlideshowDirection options, uint slideshowTick);
      [PreserveSig]

      uint GetSlideshowOptions(out DesktopSlideshowDirection options, out uint slideshowTick);

      void AdvanceSlideshow([MarshalAs(UnmanagedType.LPWStr)] string monitorID, [MarshalAs(UnmanagedType.I4)] DesktopSlideshowDirection direction);

      DesktopSlideshowDirection GetStatus();

      bool Enable();
    }

    public class WallpaperWrapper
    {
      static readonly Guid CLSID_DesktopWallpaper = new Guid("{C2CF3110-460E-4fc1-B9D0-8A1C0C9CC4BD}");

      public static IDesktopWallpaper GetWallpaper()
      {
        Type typeDesktopWallpaper = Type.GetTypeFromCLSID(CLSID_DesktopWallpaper);
        return (IDesktopWallpaper)Activator.CreateInstance(typeDesktopWallpaper);
      }
    }
  }

  public class DPI {  
    [DllImport("gdi32.dll")]
    static extern int GetDeviceCaps(IntPtr hdc, int nIndex);
    [DllImport("gdi32.dll")]
    public static extern IntPtr CreateDC(string lpszDriver, string lpszDevice, string lpszOutput, IntPtr lpInitData);

    public enum DeviceCap {
      VERTRES = 10,
      DESKTOPVERTRES = 117
    } 

    public static float scaling(String name) {
      var hdc = CreateDC(name, "", "", IntPtr.Zero);
      Graphics g = Graphics.FromHdc(hdc);
      IntPtr desktop = g.GetHdc();
      int LogicalScreenHeight = GetDeviceCaps(desktop, (int)DeviceCap.VERTRES);
      int PhysicalScreenHeight = GetDeviceCaps(desktop, (int)DeviceCap.DESKTOPVERTRES);

      return (float)PhysicalScreenHeight / (float)LogicalScreenHeight;
    }
  }

  public class Wallpaper
  {
    public static void Main()
    {
      SystemEvents.DisplaySettingsChanged += SystemEvents_DisplaySettingsChanged;
      Application.Run();
    }

    static void SystemEvents_DisplaySettingsChanged(object sender, EventArgs e)
    {
      string path = "C:\\Windows\\Web\\4K\\Wallpaper\\Windows\\img0_{0}.jpg";

      string[] resolutions = {
        "768x1024",
        "768x1366",
        "1024x768",
        "1200x1920",
        "1366x768",
        "1600x2560",
        "2160x3840",
        "2560x1600",
        "3840x2160"
      };

      uint counter = 0;
      string wallpaper = "";

      foreach (Screen screen in Screen.AllScreens) {
        float scaling = DPI.scaling(screen.DeviceName);

        int width  = (int)(screen.Bounds.Width * scaling);
        int height = (int)(screen.Bounds.Height * scaling);

        string resolution = width + "x" + height;

        if (! resolutions.Contains(resolution)) {
          if (width > height) {
            resolution = "3840x2160";
          } else {
            resolution = "2160x3840";
          }
        }

        wallpaper = path.Replace("{0}", resolution);

        if (File.Exists(wallpaper)) {
          SetWallpaper(counter++, wallpaper);
        }
      }
    }

    public static void SetWallpaper(uint id, string path)
    {
      DesktopWallpaper.IDesktopWallpaper wallpaper = DesktopWallpaper.WallpaperWrapper.GetWallpaper();

      if (id <= wallpaper.GetMonitorDevicePathCount()) {
        string monitor = wallpaper.GetMonitorDevicePathAt(id);
        wallpaper.SetWallpaper(monitor, path);
        wallpaper.SetPosition(DesktopWallpaper.DesktopWallpaperPosition.Fill);
      }

      Marshal.ReleaseComObject(wallpaper);
    }
  }
}
"@ -ReferencedAssemblies 'System.Drawing.dll', System.Windows.Forms

[WinAPI.Wallpaper]::Main()