Can't find the place you choose Start Time for the image you see on the stage of the DVD - I not talking about the Start Time of the actual video - there is a place where you can change to a point in the video that is visable as the Nav menus Image.
For DVDA 3 and DVDAS 3 - click on the button, on the right hand pane, select Media and you should see the start time in the "Thumbnail Properties" section.
And, on the timeline for the media that you use for a thumbnail, you can double-click on the digital time display to the left of the timeline, and then copy that time and then paste it into the time area that Scott describes. This lets you get the precise frame from the media that you want to use as a thumbnail (assuming you use a static thumbnail).