Below the puzzle you will see a row of tiles to choose from.
Each tile contains a part of the picture. With the right and left arrows you can move along the row to show more tiles.
Click with your mouse on a tile in the row, and then click on an empty box on the puzzle. Your chosen tile will be put into the puzzle in the place you clicked on.
If you want to remove a tile from the puzzle again, just click on an empty tile space in the row, and then click on the tile you want to take out.