If you find a photo on a US federal website, it is public domain, as per an article I read just now. It also talked about the fact that a photo or drawing doesn't have to have a copyright notice on it to protect it. RedBubble and Flickr photos and drawings belong to the person who originated the work - so you'd have to ask permission from those artists. Many photos you find on Google are from Flickr or RedBubble.
If you find something on one of those sites that you want to use, you can always write to the artist and ask permission to use their image.
I'm going to email my friend who is a lawyer and ask him if he knows of easier ways to detect copyright on internet images. He's not a copyright specialist, so he might not know either.