c# - wpf - Main UI hang during execution of Thread

Your main thread hangs because of Dispatcher.Invoke wrong usage, it should be used for UI logic only, so you should move out the Ping-oriented logic out of it.

Do not use the BackgroundWorker for this, it's an obsolete and heavy construction you don't really need. Also, do not a thread for pinging, this is wrong approach, and that's why:

Pinging operation is network-related, and the thread you're using for wait the remote server to response simply wastes the system resources, as it does absolutely nothing except wait. So you should switch to asynchronous approach for this.

You either should subscribe to Ping.PingCompleted event and call the SendAsync method, like this:

private void ping(string IP, MediaTypeNames.Image img)
{
    Ping p = new Ping();
    PingReply r;
    // lambda delegate here, may use event handler instead
    p.PingCompleted += (sender, args) => { PingCompleted(args, img); };
    r = p.SendAsync(IP, 1000, new byte[5], null);
}

private void PingCompleted(PingCompletedEventArgs args, MediaTypeNames.Image img)
{
    this.Dispatcher.Invoke(() =>
        {
            string imageAddress;
            if (args.Reply.Status == IPStatus.Success)
            {
                imageAddress = "subonline.gif";
            }
            else
            {
                imageAddress = "suboffline.gif";
            }

            var image = new BitmapImage();
            image.BeginInit();
            image.UriSource = new Uri(imageAddress, UriKind.Relative);
            image.CacheOption = BitmapCacheOption.OnLoad;
            image.EndInit();
            ImageBehavior.SetAnimatedSource(img, image);
        });
}

or you should use async/await feature which was introduced exactly for such cases (code partially from this answer):

// async event handler
private async void btn_Click(object sender, EventArgs e)
{
    // async call to all the ips
    var results = await PingAsync(new List<string> { IP1, IP2, IP3, IP4, IP5 });
    // here we do not need the Dispatcher as await will restore UI context for you
   PingCompleted(results[0], img_m);
   // etc
}

private void PingCompleted(PingReply r, MediaTypeNames.Image img)
{
    string imageAddress;
    if (r.Status == IPStatus.Success)
    {
        imageAddress = "subonline.gif";
    }
    else
    {
        imageAddress = "suboffline.gif";
    }

    var image = new BitmapImage();
    image.BeginInit();
    image.UriSource = new Uri(imageAddress, UriKind.Relative);
    image.CacheOption = BitmapCacheOption.OnLoad;
    image.EndInit();
    ImageBehavior.SetAnimatedSource(img, image);
}

// helper method
private async Task<PingReply[]> PingAsync(List<string> theListOfIPs)
{
    Ping pingSender = new Ping();
    var tasks = theListOfIPs.Select(ip => pingSender.SendPingAsync(ip, 1000, new byte[5]));
    return await Task.WhenAll(tasks);
}

Do not include the call to p.Send method in the dispatcher - currently only the Thread.Sleep(500) is done in the background and everything else is done in the UI thread.

Also, I believe that because of the Thread.Sleep the function never gives the UI thread a chance to work so the pinging never occurs. Replace the use of Thread.Sleep with a Thread.Timer object.