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All works on Wikimedia Commons are required to be released under a free license by their copyright holder. Unless the uploader is the copyright holder, we require proof that the copyright holder released the work under the license. This is typically done either by submitting permission via e-mail, or by posting a public statement on the copyright holder's website.
Sometimes, users will upload an image from a website that claims to release it under a free license, but in fact that website is itself using the image without permission (and often without any attribution!). This is called license laundering. This is particularly common with photo sharing websites such as Flickr or Picasa Web Albums that allow their users to specify a free license for their images. Such uploads may evade detection as copyright violations, since the source website appears to provide "evidence" for the license.
If license laundering is suspected, even if the source user is claiming to be the copyright owner, the file should be nominated for deletion.
Because historically this has happened most frequently with Flickr, the term Flickr washing to refer to license laundering via Flickr is also commonly encountered.
Detecting license laundering
There are several ways to detect license laundering. One of the simplest is to search for the image—using keywords from its title/description or using an image search such as TinEye or Google Search by Image—and see if an official source website can be found. Usually laundered files are available elsewhere on the web, although they may also be scanned from paper sources.
Another simple technique is to look at the other photos included in the same page or set at the source website. Look at EXIF metadata if it is available. Were the images taken at about the same time, using the same camera, in the same location? Do they have about the same resolution and exhibit the same artistic style or level of quality? Or are they all dramatically different? A hodgepodge of images points to reproduction of others' images without permission, while similar images suggest legitimacy.
Examine the source website to determine who uploaded the images. On sites like Flickr, this information may be available through the user's profile information. Such information can give hints to the identity and profession of the uploader. If the file is high quality, the uploader should be identifiable as a professional or skilled hobbyist. If the file depicts models or celebrities, the uploader should be someone who works with celebrities.
Finally, it is often useful to contact the source website (e.g., message the Flickr user) and politely ask where they obtained the image in question. Often the person who borrowed the image will be happy to disclose that they are not in fact the copyright holder, and merely copied the image from elsewhere.