You might wonder how this can be proven. In simple terms, if you copy an image from within bitcointalk, where the original creator is known, like one from the "10th-anniversary art contest," and you post it here somewhere, claiming it to be your own without referencing the original poster, it's a clear-cut case of plagiarism, and I can guarantee it will result in a ban.
Read @mprep's reply above, since he's a global moderator who has hammer to ban any user, I trust his words in this case. If you still stand with your opinions, I dare to challenge you to find which user get banned due to committing plagiarism about user's image.