People often upload images and other media files to Wikipedia and to the Wikimedia Commons – photographs, works of art, posters, etc. – and claim them as their "own work" or as "self-made", and even themselves as the "author", under the idea that scanning a document they have access to, photographing some work or item (as opposed to a scene in nature), taking a screenshot, and/or the act of uploading itself, confers upon them the status of creator and owner of an already copyrighted work. With some exceptions explained below, it does not.
As an illustration of the logical flaw inherent in thinking any of these acts could result in the uploaded file being your "own work", consider this: If you were to take a book off your shelf and manually re-type it, would that make you its author and the owner of its copyright? Of course not.
Nevertheless, many users, laboring under this misunderstanding, purport to release the copyright of such images, owned by someone else, under one of the free copyright licenses this project accepts (or into the public domain) – claiming the file as their "own work", when the image was never theirs to begin with.
As a simple example of valid ownership, you are the owner of an (original) painting that you painted (or inherited from the painter). Photographs can be less straightforward. As a general rule (outside of legal inheritance), you are not the owner of a photograph unless you took the photograph with your camera, or paid for it to be taken in a work for hire relationship. But that is only the first step in analysis, because, taking a simple photograph of a two-dimensional work that is already copyrighted, creates no ownership in you. For example, if you snap a flat photograph of a document (or merely scan or download it from somewhere), you have done nothing original and you do not own the copyright to the resulting file.
It becomes more complicated where the manner of your reproduction results in significant original transformation, modification or adaptation of the already copyrighted work you photographed or reproduced. When that is the case, a secondary copyright may be created, called a derivative work. Nevertheless, that part of the picture depicting the existing copyrighted work, retains its copyright. This means, in turn, that you cannot release that work unless you also own the copyright of the original work. Note that original photographs that contain only a very insignificant part of an already copyrighted work may meet a "de minimis" exception to copyright protection.
Please do not claim ownership of other people's work as your own and then attempt to release their copyright over which you have no authority. Doing so is copyright infringement.