While it has been impossible to set your own poster frame (unless you are a YouTube partner), YouTube does offer three thumbnail options and lets you pick the image that you want to set as the default thumbnail for that video.
Here's how it works - if your video clip is x seconds long, YouTube will generate three image thumbnails at x/4, x/2 and 3x/4 seconds. Video publishers can pick the frame that best describes their video.
To set a custom poster frame in YouTube, click the "edit video info" button next to your video description (you should be logged in). Pick one of the three video still images and click save. That's it.
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