It's of a Nomura jellyfish, and the article has this to say:
Nomura jellyfish make up the majority of the population and are one of the largest types of jellyfish. They can grow up to two meters in diameter and weigh up to 220 kilograms.The photograph is attributed to Yonhap, though Gangwon Notes wonders whether its real. I looked into it, too, and did a Google image search for "huge jellyfish" and came to this post on a blog named Cakehead Loves Evil which has the same debate in the comments section . . . and a great collection of jellyfish photos. A few seconds more and I found the "Real or Not" debate among divers on Divester.com, with the consensus being "not." I'll give the final word to the site Museum of Hoaxes, which says "probably fake." The Photoshopped jellyfish photograph is the second result for a Google image search for "Nomura jellyfish," so that's probably how that worked. For comparison's sake, here's another photograph of a diver and the business end of a Nomura jellyfish. The article even says "Sea of Japan," right in the first damn sentence, so case closed.