Hey, I'll just jump in here. :'D
Yes, you are violating intellectual property when you do this sort of thing without someone's expressed permission. IP law is complicated, so I'll break it down for you:
Giving credit isn't legally enough.
Some people are cool with it all if you just give credit, but if people don't want to have their work traced, you cannot publicize any tracings of it.
They retain legal rights over their property, and they alone set the standards of what is acceptable use of their work. And unless they state that you can reference their work (often under certain conditions), you may not trace. It is a crime, because you are infringing on their intellectual property.
Think of it this way. Your friend has a car, right? But unless he gives you permission to drive it, you can't drive it. If you take it, even if you plan on returning it, it is still theft.
Source: I'm studying Intellectual Property law, as I do plan to take it as a profession. ^^