If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
You can select from Easy, Medium, Hard, Expert levels. The difference is in the number of pieces in which the puzzle will be shuffled before you start to place the peices in their places. The modes differ from 12 pieces on the easy mode, 48 for the medium menu, 108 pieces for the hard level and 192 pieces for the hardest, the expert level! You'll get better each time you make an effort! Have fun!