The link would only appear if you insert the link directly from the website into the image tag, but if I were to download the same image and upload it on some other platform like imgur, it will not show any link to the original source.
I have seen members do that a lot. They upload an image to imgur to take advantage of imgur's quick resizing options. As you know, Bitcointalk doesn't have resizing buttons available when you create new posts. You would need to know the codes by heart to resize your images properly. Imgur, on the other hand, allows you to use the original size as well as different thumbnail and square sizes.
Plagiarism does apply to images cause it can be stolen, but it's difficult to prove, probably the reason why it is not strictly enforced.
Like I said in my previous post, I wouldn't consider posting an image as an act of plagiarism unless it's clear from the post that the author is trying to make the public think the image or artwork was created by him.