You will have to use ffmpeg or similar tools on the server with PHP exec function.
exec("ffmpeg -ss 00:03:00 -i VideoFile.avi -frames:v 1 out1.jpg");
This example will produce one image frame (out1.jpg) somewhere around the third minute from the beginning of the movie
Then you can use the generated image to show as preview image (use flowplayer or any similar video players which support showing a preview image)