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 by following these steps:
Step 1: Insert your video
Go to "Insert" > "Video" > "Video from File"
Step 2: Add a placeholder from your computer or select a frame of the video as a placeholder
2.1 Add a placeholder from your computer
Select the video file in your PowerPoint > open the menu "Format" > click on "Poster Frame > "Image from File" > "Image on this computer" > select your image and press "Insert".
2.2 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 > Select the video file in your PowerPoint > open the menu "Format" > click on "Poster Frame" > "Current Frame".