So i was curious about this…just looking for some clarity. If you start out with someone else’s base mesh (say something that is included on a tutorial dvd) and spend time testing some things out and eventually create something that looks nothing like the original, can that image still be used in promoting yourself? or is it still considered some sort of infringement since it’s not ALL your work???
technically, if you used it in zbrush and subdivided and such, it would not be recognizable and therefore you could live guilt free. on the other hand, its probably not a bad idea to understand how to make a base mesh and really should take that much effort in another 3d editor and a little study
Of course it’s up to the creative ethics of the individual, but, if your looking to promote yourself, you need to be as honest and open as possible about your artistic process. If the image goes into a demo reel, you should at least give mention, or some sort of note that, while the sculpture is your original concept, the base mesh it was done on is not. Failure to mention that, might come back to bite you in the vitals one day, especially if your hired as a modeler, and your boss asks for a base mesh. 
Of course, Picasso was able to take a child’s toy car, stick it on a pedestal. and call it his sculpture. Though, even in this case, his intent and process was transparent and accessible to everyone.
Collage artists also take from sources they did not initially manufacture. But, their final work is always their own, and nobody can viably claim or sue them to the contrary.
Also, since all ZBrush users build in software copywrighted by Pixologic (true of any software or digital art in general), really NOTHING is 100% derived by the image maker. Everything is an intense collaboration of external sources.
So, full disclosure of artistic process is the best route to follow, I think, to avoid potentially lethal misinterpretations from the audience.
Revealing how you work will also make you a better artist in the long run, as you’ll be able to admit to and concentrate on improving whatever areas you are not satisfied with.
Good luck!