To be honest, I've never seen ANY old medieval paintings where a sword was scabbarded on the back. Think about it. If it was really that big, it was belted to the horse. The physics of drawing such a weapon would be impossible, your arm just isn't long enough to pull it clear of the scabbard and still keep your hand on the hilt. I think that perhaps those big, conan-type swords with the scabbard on back, is a Hollywood thing. And we all know just how important Hollywood has made it to "get it right" over the years.

 Don't sweat the petty things, and don't pet the sweaty things!  ; )