One idea could be some middleware like:

/// <summary>
/// Replaces the original login background image.
/// </summary>
public class LoginBackgroundMiddleware
{
    private readonly RequestDelegate _next;

    public LoginBackgroundMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        if (context.Request.Path.StartsWithSegments("/Util/images/login/login-background-image.png"))
        {
            context.Response.Redirect("https://images.unsplash.com/photo-1509822929063-6b6cfc9b42f2");
        }
        else
        {
            await _next(context);
        }
    }
}

You'd add it to the Configure() method in Startup:

// Custom login background
app.UseMiddleware<LoginBackgroundMiddleware>();

Final result:

Hi Ted

That's an excellent proposal! I'll try it out.

Thank you!