In this game you can't just pick a piece you like. Your only option is to pick any one from the top layer and than to look for where it fits. After picking a piece you can't change your mind because one you pick it up the piece litetarally gets glued to the mouse until you discover its designated spot. You just can't get rid of it. And when you do, you need to keep clicking and keep rotating it until you make the perfect match with the underlaying shape. Just keep clicking until the missing tile fits in by itself.
I think that Transformers are particullary hard for jigsaws. All the parts are alike, they are mostly metalic geometrical shapes so they look very similar to each other. One of the main differences between Bumblebee's and Optimus Prime's tiles is in their colors. Bumblebee is yellowish while Prime is in blue and red colors. Beside that, you can try to match the tile's shape with the hexagon shape of the missing tile from the picture's surface.