移动到较新位置时，PictureBox仍保留在旧位置

I'm learning C# using the Head First C# book. In the first lab when building the Greyhound Racing game, I've encountered some behavior and I'm not understanding why my code is rendering the way it is. Upon the first click of the Race button, the horses race to the end of the track, yet they are rendered such that they each create a trail of previous images behind them until they reach the end of the track whereupon the preceding images finally vanish. Upon subsequent clicks of the Race button, the same thing occurs but it also fails to wipe the PictureBox for each dog from the finish line until the current race is complete. I'm learning C# using the Head First C# book. I