How can I set a poster image using particular video frame (flv)? I don't have image file available, so I wonder if it's possible to set a poster image "in the fly" from the video source.
No, that's not possible with JW Player.
Edit: A little more explanation.... JW Player doesn't actually "play" or "process" the video in any way. It's just a steering and styling script - it feeds the video to the browser's <video> tag if the browser can handle it, or to the Flash plugin. The player provides its own control bar, advertising capabilities, and so on, but when it comes to the video file itself, the player isn't touching it. So, there's no way to do things like extracting certain frames. The player isn't ffmpeg.