I've seen that picture before, but never really investigated it. So this time I did. The jellyfish is real but the picture isn't. The jellyfish shown is one of the biggest in the world (by a small amount, the Lion's Mane can be bigger), and if you read one of the links below, you'll read about a fishing trawler that capsized when it tried to haul in a load of these things. When you see the pictures, you'll see why. Anyway, it's a Nomura jellyfish and it can get up to 2 meters in diameter -- nothing to sneeze at -- but not as big as indicated by the picture.
Japanese fishing trawler sunk by giant jellyfish
Giant jellyfish pictures: Japan's Nomura invasion (National Geographic)
How do you tackle an invasion of giant jellyfish? Make sushi (this has the original of the picture; without the diver)
This isn't a Nomura, but it's big anyway:
This is a Nomura:
This picture is kinda unreal, but I think it's authentic:
And this is what it looks like when a trawler nets a bunch of 'em: