I got this in an email today and immediately said BS!!!
Straight off the Urban Legends site
"Comments: I urge anyone who thinks this photograph is real -- or even might be real -- to visit a psychiatrist. Right away. Thanks to Photoshop, the dog pictured here (which appears to be a Neapolitan mastiff, not an English mastiff as claimed) is roughly twice its natural size.
Back in 2001, Joe Flynn, the owner of a 282-pound English mastiff named Hercules (see photo), was indeed told by a representative of Guinness World Records that he had the world's largest dog. This is not the same animal. "
http://urbanlegends.about.com/library/bl_hercules_worlds_biggest_dog.htm