Each media file available on Wikimedia Commons must be under a free license or in the public domain, and should have as much information about its copyright status as possible. Specifically, besides the licensing template, you should make sure that other people have enough information (source and/or creator data) to verify the claimed copyright status.
Source
Where did the file come from?
In order to make sure that the claimed copyright status is accurate, the file description page needs to mention not only who generated the file/ image (which should already have been provided in the author= field) but where the file in question came from. This is known as its "source" and it allows others to verify that the licensing is correct and the file meets the licensing criteria to be used by anyone, for any purpose, even commercially. Source information should be provided in the source= field of the {{Information}} template:
If you are uploading a file or image that you have created entirely by yourself, you should mark the source field as source={{own}} (this is done for you if you use the upload wizard and select "This file is my own work" during the "Release rights" portion of the process); if you are uploading your own photograph of someone else's work, you should use source={{self-photographed}}; if you are uploading your own image that is based on other images already on Commons, you should use the {{own based}} template like this: source={{own based| 1= |2=}} and after the 1= you should put the name of the first image that you based your own image on (e.g., 1=Myphoto.jpg); if you used a second image as well, place the name of that image after 2=, otherwise you may leave this part of the template blank.
If you are uploading a file that originally came from an external website, you should provide at least a link to the web page (i.e., the url) on which the image or file is displayed (that is, not the web address of the file itself, but rather the web page containing the file). If that page does not also fully explain the freely licensed copyright status of the file, you should also provide a second link to a related page that discusses the website's content copyright, such as its terms of use. Note that it is entirely possible for different versions of what appear to be the exact same image to have been released under different licenses, so tread carefully.[1]
If you are uploading content that you are certain is in the public domain by virtue of its copyright having expired, it will be important to know the date the content was first published (as entered in the date= field) to assess this. Information on the death of the creator may also useful. In cases of art which is many centuries old, this is obviously less important than for, say, a 20th century photograph (also, centuries old artwork should use the {{Artwork}} template rather than the {{information}} template).
For files transferred from another Wikimedia project such as Wikipedia, see Commons:Moving to Commons.
Licensing
What is the copyright status of the file?
A licensing template must be included on the file's description page in order to ensure that the file is freely usable and modifiable under our licensing criteria ("by anyone, for any purpose, even commercially"). If the file's description page doesn't contain a licensing template, then we can't be sure if the file meets our criteria; it might in fact be a copyright violation. Furthermore, it's important for potential reusers to know how to properly reuse the file – some licenses, for example, require one to name the copyright holder of the file ("attribution") and to apply the same license to derivative works ("share-alike"). Licensing templates are usually placed below a == {{int:license-header}} == section header and above any [[Category:]] links.
Notes
↑For example, a photographer may release a low-resolution version of a photograph under a public domain license, while retaining full copyright protection to a much higher resolution version of the same image.
