@3dc Critique, suggestions and general feedback is not considered teaming. In fact, we encourage giving feedback to others, as that's the spirit of our community!
But you are able to only use what you created.
Let me give you an example, if someone did an overpaint of your picture, you cannot take that overpaint, paint on top of it and submit it as your final picture. However, you can learn from it, adjust your original picture yourself and continue with that - that way everything in the final picture is painted by your hand, even though you based your painting on someone's feedback.
Same with thumbnails, an artist can take the inspiration, but they need to create the thumbnail themselves.
Hope that cleared it out, if you have any further questions, feel free to ask more!