Hey Alexandra,
We don't really know exactly which frame has a picture, and this is why we always try to get the first frame of the video. If you want, you can edit the file /administrator/components/com_easysocial/includes/video/video.php and at line 1905, replace it with the correct frame.
Having said that, if you set the value to 00:00:10, if the video has a blank screen on the 10th frame, you will hit the same problem