Move the image blocks in adjacent image in the empty slot by just clicking on them. Rearrange all blocks so that they are placed in their original position. After completing the picture, you will know more about the image e.g. where was this image taken, which fort, what more can be seen over there etc.
For a reference while moving the blocks, you can anytime click on the Original button to view original image. When you see the original image, click again on the same button to resume moving the blocks.
Happy unblocking !