Agreed with Mut@t@ on this one with a bit of a point. The training only has two bells. So SOMEONE had to be tied to the tree. It's rather obvious they were all skilled enough to get the bells. From the third hokage no less. Thus they're legendary. But with only two bells, Jiraiya was obviously the logical candidate to be tied to the tree. We all know Orochimaru was far too sasuke-like to be tied to it, and tying Tsunade to the tree would be like pre-teen bondage. *shudder*