Ok here is a little on this that i had given to me from another site on some skies a person was selling as their own but i could see that they were mostly bryce 5 skies tweaked.

After contacted someone from the site i was told that if a person spends 5 minutes at tweaking a sky then it is infact theirs to do with it whatever they please. Now the message i got for this was long and drawn out so i just cut it down for here and by the way this was last moth so i no longer have that reply.

Therefore i believe that this would also go for textures, if one spends 5 minutes or 5 hours tweaking a texture then infact it is theirs no matter who makes the original texture.

If a person modifies a model that someone else spends hours making and the one that modifies it in way that it no longer looks like the original model then infact it can be called their own but if they modify it and you can still distinctively pick out the original model then they can not state it is theirs without stating it was made by "Name of original modeller" but tweaked by the person using it. They however may not sell or give away that model since infact it is not really theirs  

I have opinions of my own -- strong opinions -- but I don't always agree with them.