First of all you make a screenshot of the game. You can read how to make a screenshot here:
http://www.gamershood.com/forum/showthread.php?t=6433

Then you upload the picture using the manage attachment option. You can find a detailed describtion about it here:
http://www.gamershood.com/forum/showthread.php?t=10937

Please post again, if something is still unclear.