Sleep task (System.Threading.Tasks)

I need to create thread which will replace photo in Windows Forms window, than waits for ~1second and restore the previous photo.

I thought that the following code:

TaskScheduler ui = TaskScheduler.FromCurrentSynchronizationContext();
var task = Task.Factory.StartNew(() =>
{
    pic.Image = Properties.Resources.NEXT;
    Thread.Sleep(1000);
    pic.Image = Properties.Resources.PREV;
}, CancellationToken.None, TaskCreationOptions.LongRunning, ui)

do the job, but unfortunately doesn't. It freezes main UI thread.

That's because it's not guaranteed that there is one thread per one task. One thread can be used for processing several tasks. Even TaskCreationOptions.LongRunning option can't help.

How I can fix it?