Articles tagged as: Wikimedia.
Wikipedia is a popular source and possibly currently the largest repository of data. Despite its success, several teachers, professors and other academics discourages if not outright forbid students from using Wikipedia as reference material. There are reasons not to trust the credibility of Wikipedia and many of these reasons are from its original design.
The websites Wikipedia and Wikimedia Commons are excellent source of public domain images. However, as the websites that anyone can edit, it is plagued with incomplete and contradictory licensing statements that reduce your choices to either exclusively using public domain images or contacting the copyright holder. The first option is easier but you still need to weed them out from among the many copyrighted images and ensure that they are indeed copyright free.