It's quite easy to change the poster frame of a video in PPT.
Open your presentation and select the video by clicking on it.
Now let the video play, stop when you reach the spot where you want to use the frame and pause the video. Or just click on the spot in the media bar where you want to use a frame. Remember you can use the forward and backward buttons on the video bar to advance/rewind by 0,25 seconds.
Click the Video tools tab. In the Adjust group click on the Poster Frame button.
Choose Current Frame and you are done.
If the frame is not what you had in mind go back to the Poster Frame button and choose the Reset option which reverts the video to the very first frame.
If you want to use a totally different poster frame even one that does not appear in the video click the Poster Frame button and choose Image from file. Navigate to the picture you want to use and click Insert.
