According to chinese rules handicap stones are placed anywhere. According to japanese rules handicap stones are placed following a given pattern. If nothing is explicitly agreed upon, most players will expect black to use japanese rules. The following list gives the handicap stone positions on the 19x19 goban. This is also explained very well on Wikipedia at: http://en.wikipedia.org/wiki/Go_handicap

• 1 stone: The weaker player plays black without komi.
  
• 2 stones: D4 + Q16
  
• 3 stones: D4 + Q4 + Q16
  
• 4 stones: D4 + Q4 + D16 + Q16
  
• 5 stones: D4 + Q4 + K10 + D16 + Q16
  
• 6 stones: D4 + Q4 + D10 + Q10 + D16 + Q16
  
• 7 stones: D4 + Q4 + D10 + K10 + Q10 + D16 + Q16
  
• 8 stones: D4 + K4 + Q4 + D10 + Q10 + D16 + K16 + Q16
  
• 9 stones: D4 + K4 + Q4 + D10 + K10 + Q10 + D16 + K16 + Q16
  

In the waiting room and on the invite page there is an option to restrict a game to Standard handicap placement. This follows the outlined patterns above. For the other board sizes and more handicap stones similar patterns are used. On the 19x19 goban, the following pattern is used.
 
The Standard handicap placement is available for

• 19x19 boards
  
• odd size boards above 7x7: up to 9 stones
  
• other boards: up to 4 stones