Exported Poster Frame not setting to my choice

All of the instructions I find for making Poster Frames have no influence on what represents my video once it is uploaded to Dropbox. They just change the poster frame for items in my Media Browser repository for this project.

I need my video file to display in Dropbox (or other storage sites if possible) with what actually started my exported video - a  jpg image file, and NOT the first frame of the imported video that follows it on the timeline.

Is there some way to force the thumbnail of an exported video to what I want?

Thanks