As you know, any celebration in Japan has an interesting symbolism and the koinobori is no exception: because it swims against the current, the carp symbolizes strength and perseverance, so by raising carp banners, the parents wish for their sons to grow strong and willing to surpass any obstacle...
The color, the shape and the positions of the carps, they all have a meaning too: the largest carp, colored in black, represents the father, the second carp, colored in red, is the mother, and then there's a different colored carp (blue, green) for each boy. Also, the first banner isn't shaped like a carp, it represents a flying dragon, because an old legend tells the story of one carp that swam up a waterfall and became a dragon...
EXIF info:
Nikon D90
Lens: VR 18-55mm F/3.5-5.6G
Focal Length: 48mm
Aperture: F/5.6
Shutter Speed: 1/1000s
ISO Sensitivity: ISO 200