I'm holding a photo contest. How to tell whether the photos he sent were taken with his own camera or downloaded from the Internet?

It's very simple ~ you can refer to EXIF information (original information of pictures) by asking the participating authors to attach the original documents. If not, and incomplete, it may be "plagiarism"? Very useful for documentaries and scenery. If it is creative, PSD format is needed. ? This method can be used regardless of whether the works sent by the participating authors are paper or electronic. Support original works!