recently I found a video on the net ( a .mp4 file), and when I had it on my computer with Windows7, I noticed its thumbnail was not the average windows 7 video thumbnail (which looks like a piece of video film with a random picture from the movie), but a custom thumbnail! Looking in the file properties did not help find the correct button to change the thumbnail...so I just wonder how he did it! Here is a picture: left: the custom thumbnail, right: the average thumbnail...