From Joomla! Documentation

Warning sign This image has an unknown copyright status and/or is unused. Unless a source is given, an author is stated and the copyright status is provided with a free license, the image may have to be deleted without advice. Also if an "unknown" image is not useful for at least one topical page on, it will be deleted a couple of days after this warning was added. - Please read the upload formular carefully!
We also have a list of accepted licenses on this Wiki.