@SrWebDeveloperDec 23.2009 — #Client side - screen snapshot, create thumb after scaling down.
Server side - Here's a class which basically browses to a specified page (could be your own site) and takes a snapshot of it. You then make a thumbnail using a script like this. Maybe someone else has a "one step" script, but these two scripts are top quality and work well, easy to implement.
Remote - visit http://www.remote-screenshots.com/ where you can create the screenshot of whatever page you want and it will scale it down partially, then you scale it down further manually. Works okay, not super reliable.
If you want something specific to Flash, I could not find a tool that handles this easily - but it sure is easy to screenshot/scale/create thumb - maybe in your ActionScript call PHP via an Ajax call and process it that way. You get the idea, I hope. If not, I'm clueless otherwise! heh
@MindzaiDec 23.2009 — #I would guess the best way to do this is to have your flash game take a screenshot of itself and POST it to a PHP script as mentioned above. Otherwise it's going to be difficult getting a screenshot of just the game content without doing it manually (which I assume you want to avoid).