While works are now automatically copyrighted once they're put into a tangible form, authors can choose to add licenses encouraging reuse. Creative Commons is an organization that facilitates this by offering creators a series of easy-to-understand, custom licenses they can apply to their works. This also means that there is an increasing amount of new work that anyone can copy and re-use without asking for permission.
Note: Because Flickr, Google, and other search engines rely on web crawlers and user-generated content, there are sometimes errors in the labeling of usage rights. It is your responsibility to do further research if you encounter an object you think may be copyrighted (for example, if it has a watermark or says all rights reserved).