If it is not the first frame or the last frame, it is almost a matter of luck to get a good thumbnail. Is there a way to decide the thumbnails on my own?
it's 1/6, 2/6, 3/6, 4/6, and 5/6 into the video
in addition to picking a frame at 1/6, 2/6, etc, i believe it clips to the nearest keyframe, which is usually a multiple of 15
example, if you have 6770 frames total and 15 keyfarme intervals:
6770 * (1/6) = 1128.33..
1128.33.. /15 = 75.22.. (non keyframe, it rounds up) ~ 76 * 15 = 1140 (the first thumbnail)
you can then add/remove frames to the video to get a specific frame as thumbnail you want