I do alot of video producing, and from what little you've said, Im going to guess that the video needs to be edited. Whats happening is the very first frame is whats showing in the "preview" and right now, that frame is black, probably the begining of the title or something. Some viewers preview somewhere in the middle of the video, some, the begining. If you dont have access to the file with the original video, that which you can edit, you may have to find a different player. If you do have the file, just put it in a video editor like Windows Movie Maker and remove the first few frames so that there is no black frame before the video starts. If your looking for a certain frame to show, copy and paste that frame (1 frame) at the begining of the clip and either copy it a few times or edit it to run in slow mo for a second. Hope this helps!