Vista - How to set the thumbnail for a video??

Hi

I need some help with Movie Maker. I created a video with a few picture files. When I published it to my system, the image displayed in the thumbnail view of the video file was not the first, but a later slide of the storyboard. Not that I usually care, but this video is meant to be a surprise for a friend, and I hope you will agree that it is pretty annoying when a image to be displayed later is shown as a thumbnail in the folder. It spoils the very element of surprise!

Can anyone please let me know how to change this so that my first picture is displayed in the thumbnail?

I've been trying to find a solution to this for a long time... I'm not able to find any information on this.

Thanks!