On Windows XP and Mac, this problem never occurs, and the picture zooms smoothly.
But with Window Vista and 7, the zooming isn't smooth at all, and this kind of problem may happen.
In the past, I've spent a lot of time trying to fix this glitch, but nothing worked.
After reading your message, I decided to give it yet another try...
And guess what ? This time I've had THE idea that worked !
With the next version, the zooming will be smooth even with Vista and 7