First: I think you may find some images without source reference if you search back my oldest posts. But just like in this case, I don't think the user who posts it claim is as their own work. Just to be sure, I nowadays always add a source link to images, but not all users do that. Especially in the Wall Observer thread I've seen many (innocent) cases.

Plagiarism isn't the same as a copyright violation. Plagiarism can get you banned, copyright violations can get you into other problems. Either way, plagiarism is handled by forum rules, and shouldn't be used for negative feedback: