Posting images on the forum aren't given the same attention as its done to written works. Possibly for few reasons which could range to ignorance and users on the forum not taking it up. To be straight, I don't give it much attention either but, should you feel you are doing it wrong or there could be something wrong, you could save yourself the stress of having to think about what could happen by making the links available or stating it out in your thread.
Sometimes, it's better you just get to avoid what could go wrong than trying to solve it when it does because, you just might not find a resolution.
True, but it is better to give the source link of any artwork or articles to give credit to the creator. Besides, it isn't his work so it is ethical to give the source where he got the image. And if a problem arises from the picture at least he is just reposting it and is not the one who created the images thus saving him the troubles of any legal problems and at the same time saving him from the worry of the possible ban in this forum due to plagiarism case (though it has been cleared that images are exempted).
Though I wonder why you are asking it @OP? As you already know that we need to give the link to materials like images, articles, and other stuff that is not originally created by us.