Your placeholder image is by default the first frame of your video. However, this is often an unusable image to be displayed as the placeholder. Luckily you can easily select or upload your own placeholder - after you have inserted your video.
There are 2 options:
1. Add a placeholder from your computer
Click on the video file you have inserted in your PowerPoint.
Then click on the 'Format'-menu that appears on top.
Click on the 'Poster Frame' dropdown and choose 'Image from File' > 'From a File'.
Now you can select the image you want to use and and click 'Insert'.
2. Select a frame of the video as a placeholder
Play the video in PowerPoint and pauze on the frame that you want to make the placeholder from.
Then click on the video file in your PowerPoint so that the 'Format'-menu appears on top.
In this menu, click on 'Poster Frame' > 'Current Frame'.