See https://ithoughthecamewithyou.com/post/crushing-pngs-in-net

Crush.cs

public static void CrushAllImages()
{
    try
    {
        string appDataRoot = HostingEnvironment.MapPath("~/App_Data");
        if (appDataRoot == null)
        {
            return;
        }

        DirectoryInfo directoryInfo = new DirectoryInfo(appDataRoot);
        FileInfo[] pngs = directoryInfo.GetFiles("*.png", SearchOption.AllDirectories);
        foreach (FileInfo png in pngs)
        {
            CrushImage(png.FullName);
        }
    }
    catch (Exception ex)
    {
        //...
    }
}

public static void CrushImage(string fullPath)
{
    if (string.IsNullOrEmpty(fullPath))
    {
        return;
    }

    try
    {
        string markerPath = Path.ChangeExtension(fullPath, ".cng");
        if (File.Exists(markerPath))
        {
            return;
        }

        string crushExe = HostingEnvironment.MapPath("~/App_Data/pngcrush_1_7_77_w32.exe");

        ProcessStartInfo psi = new ProcessStartInfo(crushExe, string.Format(CultureInfo.InvariantCulture, "\"{0}\" \"{1}\"", fullPath, markerPath));
        psi.UseShellExecute = false;
        psi.CreateNoWindow = true;
        psi.LoadUserProfile = false;
        psi.WorkingDirectory = HostingEnvironment.MapPath("~/App_Data");

        Process p = Process.Start(psi);
        if (p == null)
        {
            throw new InvalidOperationException("No Process!");
        }
        p.WaitForExit();

        if (File.Exists(markerPath))
        {
            if (p.ExitCode == 0)
            {
                File.Copy(markerPath, fullPath, true);
                File.WriteAllText(markerPath, "Processed");
            }
            else
            {
                SiteLog.Log.Add(LogSeverity.Error, "CrushImage Failed (non-0 exit code) for " + fullPath);
                File.Delete(markerPath);
            }
        }
    }
    catch (Exception ex)
    {
       // ...
    }
}