"Tile" creates multiple copies of the current image in a tiled pattern. You need to set the size of the tiled image as a multiple of the current image to get a cleanly tiled image — for example, setting 400 x 400 for a 100 x 100 image would make the new image contain 4 copies of the current image.
It is often useful to select Create New Image so that you current image remain unaltered.