We must be talking about two different things. I thought that the question had to do with what shows on the menus as "previews" of the chapters.
I thought that is what we were talking about.
As far as I know, PD plays the first 15 seconds of the start of the video on the timeline in the Main/Root menu.
What I have noticed, if you have a bit of black at the start of the video, the image in the thumbnail is black.
It also plays the first few seconds of video at the chapter points in the Chapter pages.
Generating those 'clips' is part of the the time it takes to do the first 21% of the Create Disk burning.
You can control where you set your chapter point in the Edit Module. That would have some effect on the thumbnail playing for that chapter.
Fact is there is no good way to control what is shown in the thumbnails of the menu.
Carl312: Windows 10 64-bit 8 GB RAM,AMD Phenom II X4 965 3.4 GHz,ATI Radeon HD 5770 1GB,240GB SSD,two 1TB HDs.