Plagiarism is the "wrongful appropriation" and "stealing and publication" of another author's "language, thoughts, ideas, or expressions" and the representation of them as one's own original work. Wikipedia.

I no longer post images of my artwork here, nor on any other site. About a year ago I chanced upon an online advertisement featuring a product logo that was a direct copy of my work. Fortunately the logo was removed after I contacted the company, compelling them to cease & desist from using the image.

I was lucky....However, proving Intellectual property ownership can become very murky & expensive. Protect yourself by keeping concise records of everything you post online....I know I do.