Choose video-thumb by second.
Videothumb is a module for Gallery 3 which will allow the user adding a video to choose what time from the movie the thumbnail is generated.
This module may be downloaded from the codex here.
To install, extract the "videothumb" folder from the zip file into your Gallery 3 modules folder. Afterwards log into your Gallery web site as an administrator and activate the module in the Admin -> Modules menu. You will then get a new video action menu item to choose the thumbnail for the movie.