i am pretty sure that if a texture is free, you're allowed to do anything with (and to) it except for profit off of it. it is not plagiarism unless you attempt to sell/trade with your version of the free head or claim that the work is your own.
imo though, you should be able to sell it if the changes are significant enough. it could be similar to public domain laws. afaik your additions to public domain works are protected by copyright in the US if they meet fairly arbitrary standards of "originality and creativity", generally comprising things like annotations, illustrations, translations, remixes, etc.
for example, if you create
this head but you started with
this head as a base, i see no real reason why you shouldn't be able to sell your creation/additions.
that said, your changes would probably not meet the standards for resale. the 8-bit error thing looks pretty interesting though, at least on the flat
anyway, i don't think this is plagiarism because you are not attempting to sell it, you have not claimed it as your own work, and you have provided the source.
however, the rules regarding plagiarism should probably be more clearly defined. atm the rules in the art board just say "don't plagiarize" and this is the closest thing to covering the subject in the global rules:
A) Using copyrighted work without proven owner consent (plagiarism) is strictly forbidden, and you must credit all other artists work. In other words, if you're selling c/p textures or images post a link to the original image.
B) Do not sell variations of the same texture to multiple users in any circumstance, re-colors or minor edits do not constitute a new texture.
part A could be interpreted as saying that using a non-free texture as a base without permission from the original artist is not allowed and would be considered plagiarism.
the bolded phrase in part B, on the other hand, mentions that
minor edits do not "constitute a new texture". this could be interpreted as implying that
major edits could be, in fact, considered an entirely new texture.
food for thought
tl;dr — there is not much to say about your head but it does bring up some interesting points about what constitutes the creation of new and saleable art in the context of toribash