If you are refering to the image that displays behind the play button, you can't set that. That image is generated during the "processing" period after upload, before you can play it. It typically is a still in the middle of the video. If you've noticed, a lot of trap videos (rickrolls) will put a single frame of video of whatever the trap is for. Then that picture appears in the preview and doesn't actually (1 frame is too fast) show up when playing the video back. Unfortunately, unless you are the video creator, you can't specify what frame it shows. I hope I explained that well.