tl;dr Copy
right: If it exists, and you didn't make it, taking it is probably stealing under a strictly technical point of view.
That being said, most people don't tend to care if it's used in a non-profit manner, and you can get a legal exception via
Fair Use in the United States. (Other countries typically have similar laws to Fair Use. In the Commonwealth, it's called
Fair Dealing.) This exception lets you use it for a small variety of purposes, but mostly fall under public criticism, education, and research.
Ergo, if you take a picture, and someone asks you to take it down, and you aren't using it for a fair use purpose, by law, you have to take it down. This is typically done through a DMCA Notice.
Final Thought: Using pictures for stuff like characters in a role play isn't really morally wrong. Legally? Yes, it's probably wrong. Morally? Not really.
Though, you probably shouldn't take a copyright notice off of a picture. That doesn't make it any less copyright infringing and it, in all honesty, is kind of scummy when someone is obviously trying to inform you that they don't want you copying their works for your own purposes.