We need to respect the rights of the owner and always give them credit for their work. Most of them will not like it if we alter their work without their permission. I guess this is how the artistics world works.
License is a MUST and don't forget a model release. THIS is important... so you're doing everything legally: even if you take a picture of a friend and have their persmission to place it on your site, you still need a model release! There are more legal doc for minors..