image in picture control not displayed properly after moving

[dumbncool]

Hello, I'm writing a MFC application to move a picture control when i press the "Next" button. (Please see attached screen-captures)

There is a dialog box on which there are 2 picture controls. When I press the Next button, the left picture control should move on top of the right picture control.
You can see I kept the code for moving the picture control in a for loop because I wanted a sort of animation effect.

As you can see from the attachments, after I move the image there are some vertical lines appearing. How do I make them go away?

Thank you.

void CShabbirDlg::OnBnClickedButton2()
{
	// TODO: Add your control notification handler code here
	if (pageNum <= 10) {
		pageNum++;

		CRect RRect, LRect;

		//Get the coordinates of the right and left picture control
                picPreview2.GetWindowRect(&RRect);
		picPreview.GetWindowRect(&LRect);

		//Move the left picture control to the right
		for (long int i = LRect.left; i > RRect.left; i -= 20) {
			picPreview.MoveWindow(i , LRect.top, LRect.Width(), LRect.Height());
		}
	}
	

}