YouTube currently auto-selects 3 frames from your uploaded video to use as the video’s thumbnail. Knowing this, you can try to position one or more desirable frames at these exact locations:

X = Video length (in sec.)

Y = X divided by 4 or X/4 (in sec.)

Y is the 1st thumbnail.

2 times Y or 2xY (in sec.) is the 2nd thumbnail (poster frame).

3 times Y or 3xY (in sec.) is the 3rd thumbnail.