If the movie file has a custom icon, it will be used instead of generating a thumbnail with QuickLook. To set a custom icon, first Get Info on the file in the Finder. Next copy the image that you want to use onto the clipboard (Edit->Copy). Finally, select the icon in the upper left corner of the Get Info pane and paste (Edit->Paste) to replace the current icon with the one you copied. Rivet will not use it instead of the QuickLook thumbnail.