Is there any easy way to capture screenshots of specified users on the server side

As a novice, I don’t want to rely on the way I record videos because I only want to take one image

The easiest way is using ffmpeg.
You create a Consumer (over PlainTransport) and tell ffmpeg which ports the consumer is connected to.

You can use the “-vframes 1” parameter which will extract just one frame from the stream and then close the ffmpeg process.