problem in picture display

Question

i display a pic on picture box and then i want to display another pic after some seconds
but problem is that first image is not displaying only display 2nd image

1... display first picture from database such as
 pbThumbImage.Image = new Bitmap(new MemoryStream(objEmployee.Picture));

2... and then get the reference of current main thread
Thread main = Thread.CurrentThread;
Thread.Sleep(5000);

3.. and then display another pic
pbThumbImage.Image = global::TimeEntrySystem.Properties.Resources.DefaultImage;

but problem is that first image is not displaying only display 2nd image