Personally, I go by the policy that if I can't find copyright information I assume it is copyrighted and permission is needed to use it.
Wikimedia commons has a lot of photos that are "free" to use (they might still have conditions, such as attributing the photo to the author, or not modifying it, etc.)
Also, most government photos are free to use. There are some very good photos to be found from government agencies. The military and NASA for instance take some pretty good photos. They aren't necessarily the easiest to browse, they can still be a good source.
I will occasionally use a news photo because I believe that news-worthy photos belong more to all of us than to news agencies - they probably wouldn't agree with that - I don't do it often and I always attribute the source.