so for instance i can take andreyoshi's head, flip it upside down and wear it ?
No. You cannot. Hampa (yes, the creator of this game.) Has given his (silent?) permission to use and modify HIS and only HIS head texture. "Dafe's head etc" Was just proof of this. (For those who don't know it's hampa's head upside down.)
No, in most cases you still cannot. Only if the original creator/owner of the said head texure gives his/hers permission to modify/use it.
However in this case, owner/creator (hampa) has given his permission, so yes, you are allowed to use and modify his head texture as you like.