When an image is shown above another via a timer that makes it flash then it is possible to see the outline of the rectangle containing the image underneath each time it paints

When an image is shown above another via a timer that makes it flash then it is possible to see the outline of the rectangle containing the image underneath each time it paints. Since it is transparent then it should not be showing this rather it should just render the one image which is updating.

In the example attached,the indicator on the left is what is being toggled, the image under that shows the rectangle flashing then.