I always try to credit an author for anything I steal, unless I did not realize it was not canon. That could happen. Also, seems polite to ask since we do speak (usually--some do not respond to reviews), but it seems rude if the author refuses permission since we are all using Rowling's work without explicit approval, but we do give credit.

While we are on that subject, icon makers are the absolute most precious and entitled people in fandom. They demand credit for their icons like they had just painted the ceiling of the Sistine Chapel and then do not credit their source--that is some weird kind of thinking going on. I make a lot of icons, and give credit for my sources unless I cannot find them, but I do not demand credit for use of my icons. It's flattering enough for me that someone chose to copy them. Credit would be nice, but hey they are not really mine and are really, really tiny and not Fabrege eggs either; I am already using someone else's work--whether a screen cap or original art.

Icon above mine! Made it from my own screen cap! Help yourself and follow your heart on crediting!