Incorrect Thumbnail Picture when Posting a Mp4 file

I have created a "storyboard" from 25 photos using Microsoft Movie Maker. At the end of the process, I convert to an MP4 file to complete the process.
The problem occurs when I post to Facebook. I want the first photo (start of the movie) to be the thumbnail in the post. For some reason, it is picking another photo (which is 20th. in the sequence). The reason I need this is that photo 1 (start of movie) has a caption in the photo.

Any ideas?

 

I don't know for sure, but it could be that it fetches the smallest size (KB) image.

 

Thanks for the reply. I'm afraid the picture it chose is the second largest. Very annoying.